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ELEMENTS 



ASTEONOMT: 



ACCOMPANIED WITH NUMEROUS ILLUSTRATIONS, 

A COLORED REPRESENTATION OF THE SOLAR, STELLAR, 

AND NEBULAR SPECTRA, 



CELESTIAL CHARTS OF THE NORTHERN AND THE 
SOUTHERN HEMISPHERE. 



J. NOEMAN LOCKYEK, 

FELLOW OF THE ROYAL ASTRONOrHICAL SOCIETY, EDITOR OF "NATURE," ETC. 



AMEEICAN EDITION, 

REVISED AND SPECIALLY ADAPTED TO THE SCHOOLS OF TLJE UNITED STATES. 



NEW YORK: 
D. APPLETON AND COMPANY 

549 & 551 BROADWAY. 
1875. 



,A 



CHEMICAL ELEMENTS 
TO WHICH SOME OF THE 
LINES CORRESPOND. 





SPECTRA OF THE SUM. STARS. 


AND NEBULA. 








(FOR EXPLANATION, SEE PAGE 


269.) 






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D. APPLETON &. CO. N 



I%"75" 



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Published "by D. Appleton & Co. 



Quackenbos's Natural Philosophy, Embracing the most 
recent discoveries in Physics, and exhibiting the application of 
Scientific Principles in every-day life. Accompanied with 335 
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Entered, according to Act of Congress, in the year 1870, by D. Appleton 
Co., in the Office of the Librarian of Congress, at Washington. 

By Exchange 
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JANUARY 16 1934 



A* 



PREFACE. 

These Elementary Lessons in Astronomy are intended, in the 
main, to serve as a text-book for use in Schools, but I believe 
they will be found useful to " children of a larger growth," who 
wish to make themselves acquainted with the basis and teachings 
of one of the most fascinating of the Sciences. 

The arrangement adopted is new ; but it is the result of much 
thought. I have been especially anxious in the descriptive por- 
tion to show the Sun's real place in the Cosmos, and to separate 
the real from the apparent movements. I have therefore begun 
with the Stars, and have dealt with the apparent movements in a 
separate chapter. 

It may be urged that this treatment is objectionable, as it re- 
duces the mental gymnastic to a minimum ; it is right, therefore, 
that I should state that my aim throughout the book has been to 
give a connected view of the. whole subject rather than to discuss 
any particular parts of it ; and to supply facts, and ideas founded 
on the facts, to serve as a basis for subsequent study and discus- 
sion. 

It has been my especial endeavor to incorporate the most re- 
cent astronomical discoveries. Spectrum-analysis and its results 
are therefore fully dealt with; and distances, masses, etc., are 
based upon the recent determination of the solar parallax. 

The use of the Globes and that of the Telescope have both been 
touched upon. Now that our best opticians are employed in pro- 
ducing "Educational Telescopes," more than powerful enough 
for school purposes, at a low price, it is to be hoped that this aid 
to knowledge will soon find its place in every school, side by side 
with the blackboard and much questioning. 



4 PEEFACE. 

I take this opportunity of expressing my thanks to Mr. War- 
ren De La Kue, and also to the Council of the Eoyal Astronomi- 
cal Society, M. Guillemin, Mr. K. Bentley, the Kev. H. Godfray, 
Mr. Cooke, and Mr. Browning, who have kindly supplied me 
with many of the illustrations. 

I am also under obligations to other friends, especially to Mr. 
Balfour Stewart and Mr. J. M. Wilson, for valuable advice and 
criticism, while the work has been passing through the press. 

J. N. L. 



It has been the aim of the American Editor to extend the 
usefulness of Mr. Lockyer's admirable treatise, by specially 
adapting it to the schools of the United States. With this view, 
he has condensed the text in some parts, and enlarged it in 
others ; he has introduced new illustrations, and has added ques- 
tions to facilitate the labors of the examiner and to furnish the 
student with a test of his preparation. He has also extended the 
practical directions for finding interesting objects in the heavens 
on different evenings throughout the year. Celestial Charts con- 
taining the constellations and principal stars, taken from the 
"Popular Astronomy" of Arago, have been appended. They 
will be found to answer the purpose of a large Atlas of the Heav- 
ens, and will help to inspire the learner with interest in the sub- 
ject and to give a practical bearing to his studies. 

It is hoped that this treatise, embodying as it does the 
most recent and interesting results of astronomical discovery — 
many of which are due to the researches of its distinguished au- 
thor — will be found just what is needed as an elementary text- 
book on the subject, and that it may give an impetus to the study 
of Astronomy in both public and private schools throughout the 
land. 

New York, August, 1870. 



CONTENTS 



INTRODUCTION. 

General View.— Usefulness of Astronomy.— Early History of Astronomy.— Math- 
ematical Definitions, . . . . . pp. 9-23 

CHAPTEK I. 

THE STARS. 

Magnitudes of the Stars.— Their Comparative Brightness, Distances, and Diame- 
ters.— The Milky Way.— The Magellanic Clouds.— Distribution of the Stars.— 
Division of the Stars into Constellations.— The Zodiacal, Northern, and 
Southern Constellations.— Names of the Stars.— The First-magnitude Stars. 
—Proper and Apparent Motion of the Stars, ... pp. 23-36 

Double, Triple and Quadruple Stars.— Multiple Stars.— Binary Stars.— Variable 
Stars.— Mira.— Algol.— Temporary Stars.— Cause of Variations of Bright- 
ness.— Colored Stars.— Changes of Color.— Structure of the Stars.— Materials 
of the Photospheres.— Causes of Color in the Stars.— Star Groups and Clus- 
ters, pp. 36-48 

CHAPTER II. 

NEBULJE. 

Nebulae under the Telescope.— Irregular Nebulas.— Eing and Elliptical Nebulae.— 
Spiral Nebulae.— Planetary Nebulae.— Nebulae surrounding Stars.— Brightness 
of the Nebulae.— Variable Nebulae.— Distribution of the Nebulae.— Physical 
Constitution of the Nebulae.— The Nebular Hypothesis, . pp. 48-54 

CHAPTEK III. 

THE SUN. 

The Sun's Disk.— Its Distance and Diameter, Volume and Mass.— Rotation of 
the Sun.— The Plane of the Ecliptic— Inclination of the Sun's Axis.— Time 
of Rotation.— Telescopic Appearance of the Sun.— Sun-spots.— Faculae.— 
Corrugations. — Willow-leaves. — Punctulations. — Red-flames and Promi- 
nences.— Explanation of the Appearances on the Disk.— The Sun, a Variable 
Star.— Elements in the Sun.— Benign Influences of the Sun.— Future of the 
SuD > • pp. 55-70 

CHAPTER IV. 

THE SOLAR SYSTEM. 

General Description.— Explanation of the Signs of the Planets.— Historical De- 
tails.— Discovery of Neptune.— Discovery of the Asteroids.— The Suspected 



6 CONTENTS. 

Planet, Vulcan.— Motions and Orbits of the Planets.— The Satellites.— Dis- 
tances of the Planets from the Sun.— Comparative Size of the Planets.— Dis- 
tances and Kevolutions of the Satellites. — Volumes, Masses, and Densities of 
the Planets, pp. 70-81 

CHAPTEE V. 

THE EARTH. 

Shape of the Earth.— The Sensible Horizon.— Poles and Equator.— Proofs of the 
Earth's Rotation.— Foucault's Experiment.— The Gyroscope.— Imaginary 
Lines on the Earth's Surface. — Latitude and Longitude. — Zones.— Polar and 
Equatorial Diameter.— Motions of the Earth.— Velocity of the Earth's Mo- 
tions.— Inclination of the Earth's Axis. — Succession of Day and Night.— 
Length of the Longest Day in Different Latitudes.— The Change of Seasons. 
—Difference of Time and Longitude.— How to determine Longitude at 
Sea, pp. 81-101 

Structure of the Earth.— The Earth's Crust.— Stratified and Igneous Kocks.— The 
Interior of the Earth.— Density of the Earth's Crust.— The Flattening at the 
Poles explained. — The Earth's Atmosphere. — Winds, how produced.— Belts 
of Calms and Winds.— Clouds, Eain, Snow, Hail.— Chemical Elements 
of the Earth. — Composition of the Air. —Original Condition of the 
Earth, . . pp. 101— 112 

CHAPTEE VI. 

THE MOON. 

Size of the Moon.— Its Distance from the Earth.— Its Period of Revolution.— Li- 
brations.— Nodes.— The Moon's Orbit.— Earth-shine.— The Moon's Light.— 
Telescopic Appearance of the Moon.— Lunar Craters.— The Crater Coperni- 
cus.— Walled Planes and Rilles.— Absence of Water and Atmosphere.— Rota- 
tion of the Moon.— Phases of the Moon, . . . pp. 113-124 

CHAPTEE Vn. 

ECLIPSES. 

Explanation of Eclipses.— Total and Partial Eclipses of the Moon.— Total, An- 
nular, and Partial Eclipses of the Sun.— Extent of a Partial Eclipse, how 
measured.— Recurrence of Eclipses.— The Saros.— Phenomena attending a 
Total Eclipse of the Sun.— The Corona.— Baily's Beads.— Luminous Promi- 
nences.— Number of Ech'pses.— Memorable Eclipses.— Effects of Eclipses on 
the Uneducated, ....... pp. 124-135 

CHAPTEE VIII. 

THE INFERIOR AND SUPERIOR PLANETS. 

The Planets distinguished as Inferior and Superior.— Mercury.— Its Phases, Or 
bit, and Apparent Diameter. — Its Heat and Light.— Its Rotation, Density., 
etc.— Venus.— Its Size and Density.— Its Seasons.— Its Day and Year.— Its 
Heat and Light, ....... pp. 135-139 

The Superior Planets. — Mars. — Its Day and Year.— Inclination of its Axis. — Its 
Apparent Diameter. — Its Appearance in 1862.— Its Atmosphere.— Its Sea- 
sons.— Jupiter, its Revolution, Rotation, Seasons, etc.— Its Belts.— Its Rotary 
Velocity.— Probability of a Great Cloudy Atmosphere.— Jupiter's Four 
Moons.— Saturn.— Its Size, Day, and Year.— Its Eight. Moons.— Its Rings.— 
Its Seasons.— Uranus.— Its Size, Heat, and Light.— Its Four Moons.— Nep- 
tune and its Moon, ....... pp. 139-152 



CONTENTS. 7 

CHAPTEE IX. 

THE ASTEROIDS, OR MINOR PLANETS. 

Bode's Law.— Discovery of the Asteroids.— Size of the Asteroids.— Their Orbits. 
—Evidences of Atmosphere and Kotation.— Mode of detecting the Aster- 
oids.— Theory respecting the Asteroids, ... pp. 152-155 

CHAPTEE X. 

COMETS. 

General Description of Comets.— The Cometary Orbits.— Short-period Comets.— 
Long-period Comets.— Distances of Comets from the Sun.— Appearances 
presented by a Comet.— Changes in Appearance, as they approach the Sun. 
—Danger from Collision with a Comet.— A Divided Comet.— Physical Consti- 
tution of Comets. —Number of Comets. — Ccmets, bow formerly re- 
garded, pp. 155-164 

CHAPTEE XI. 

METEORS AND METEORITES. 

Number of Meteors.— The Zodiacal Light ; its Shape, and Theory as to its 
Cause.— The November Meteoric Showers.— Orbits of the Meteors.— Cause 
of the Luminous Appearance of Meteors.— Their Size and Distance from 
the Earth.— Meteoric Showers of August and April.— Detonating Meteors.— 
Meteorites.— Showers of Aerolites.— Composition and Structure of Meteor- 
ites, . pp. 164-173 

CHAPTEE XII. 

APPARENT MOVEMENTS OF THE HEAVENLY BODIES. 

The Earth, a Moving Observatory.— Apparent Movements, how produced.— The 
Celestial Sphere.— Celestial Poles and Equator.— Zenith and Nadir.— Decli- 
nation and Eight Ascension.— North-polar Distance.— The Horizon.— Altitude 
and Azimuth, . . . . . . . pp. 173-179 

Apparent Movements of the Stars.— Stars Visible in Different Latitudes.— Use of 
the Globes. — The Circumpolar Constellations.— Period of the Apparent Move- 
ments of the Celestial Sphere.— The Apparent Movements of ihe Stars, as 
affected by the Earth's Yearly Revolution.— How to identify the Stars in the 
Sky. — Constellations Visible in the United States on Different Evenings 
throughout the Year, ...... pp. 179-194 

Apparent Movements of the Sun.— Difference between the Sidereal and the Solar 
Day. — Celestial Latitude and Longitude.— Signs of the Zodiac— Apparent 
Path of the Sun.— How to determine the Time of Sunrise and Sunset with 
the Celestial Globe. — How to find the Length of Day and Night.— Apparent 
Movements of the Moon.— The Harvest Moon, . . pp. 194-202 

Apparent Movements of the Planets.— Distances of the Planets from the Earth.— 
Phases of the Planets.— Aspects of the Planets ; Conjunctions, Opposition, 
and Quadrature.— Transits.— Elongations.— Retrograde Motion, explained. — 
Synodic Period. — Inclinations of the Orbits, and Nodes.— Path of Venus 
among the Stars.— Representation of the Orbits of Mars and the Earth.— Sat- 
urn's Rings, as seen at Different Times from the Earth, . pp. 202-213 

CHAPTEE XIH. 

THE MEASUREMENT OF TIME. 

Clepsydrae.— Sun-dials.— Clocks and Watches.— The Mean Sun.— Irregularities 
of the Sun's Apparent Daily Motion.— Equation of Time.— The Apparent 



8 CONTENTS. 

Solar Day, Mean Solar Day, and Civil Day.— Sidereal Time.— The Week ; 
Names of the Days.— The Month, Lunar, Tropical, Sidereal, Anomalistic, 
Nodical, and Calendar.— The Year, Sidereal, Solar, and Anomalistic— The 
Calendar.— Old and New Style.— Change in the Length of the Solar Year. — 
Change of Aphelion and Perihelion, .... pp. 214-226. 

CHAPTER XIV. 

ASTRONOMICAL INSTRUMENTS. 

Light.— Its Velocity.— Aberration of Light.— Its Reflection and Refraction.— 
Eflect of Refraction.— Dispersion of Light.— The Spectrum.— Lenses.— Re- 
fraction by Convex and Concave Lenses.— Achromatic Lenses, pp. 226-235. 

The Telescope.— Its Invention. — Its Construction.— Its Illuminating and Magni- 
fying Power.— Eye-pieces.— The Largest Refractor. — Lord Rosse's Reflector. — 
Equatorial Telescopes.— Measurement of Angles.— The Altazimuth.— The 
Transit-circle.— Methods of determining the Time of Transit over a Wire. — 
Determination of Positions with the Equatorial.— Star-catalogues.— Correc- 
tions to be applied to Observations.— Parallax.— Changes in Positions al- 
ready determined. — Precession of the Equinoxes.— Secular Variation of 
the Obliquity of the Ecliptic— Celestial Latitude and Longitude, how de- 
termined, ........ pp. 235-255. 

Determination of Time, Latitude, and Longitude.— Determination of Distances. — 
The Moon's Parallax.— Determination of the Distance of Mars. — The Sun's 
Parallax, Old and New Value.— Parallax of the Stars.— Bessel's Method of 
determining the Distances of the Stars.— Table of the Parallax and Distances 
of some of the Nearest Stars.— Mode of determining the Size of the Heavenly 
Bodies, ........ pp. 255-263. 

CHAPTER XV. 

THE SPECTRUM. 

Gradual Formation of a Spectrum.— Fraunhofer's Lines.— Experiments with the 
Spectroscope. — Kirchhoffs Discovery. — Explanation of Fraunhofer's Lines. — 
Spectra of the Stars, Nebulae, Moon, and Planets.— Explanation of the Fron- 
tispiece.— The Star Spectroscope.— Celestial Photography, . pp. 263-271. 

CHAPTER XVI. 

UNIVERSAL GRAVITATION. 

Motion.— Tb^e Parallelogram of Forces.— Weight.— Laws of Falling Bodies.— 
Curvilinear Motion, how produced.— Newton's Discovery.— The Law of 
Gravity.— Effect of Gravity on the Moon's Path. -Kepler's Laws.— Centrifu- 
gal and Centripetal Force.— The Planetary Orbits.— Varying Velocity of a 
Body moving in an Elliptical Orbit, explained.— Gravity not Dependent on 
the Mass of the Attracted Body.— The Centre of Gravity, . pp. 272-283. 

Determination of the Earth's Density and Mass.— The Cavendish Experiment- 
Determination of the Sun's Mass.— Determination of the Masses of the Plan- 
ets.— Perturbations and Inequalities.— Precession of the Equinoxes, how 
produced.— Change in the Earth's Axis.— Nutation.— Tides, how produced.— 
Velocity and Height of the Tidal Wave.— Effect of Tidal Action on the Daily 
Rotation, pp. 283-293. 

APPENDIX.— Tables, PP- 294-299. 

ALPHABETICAL INDEX, PP- 300-312. 



ELEMENTS OF ASTRONOMY. 



INTRODUCTION. 

General Vieic. 

i. Astronomy is the science that treats of the heavenly 
bodies. 

2. The Heavenly Bodies. — At night, if the sky be 
cloudless, we see it spangled with so many staes that it 
seems impossible to count them; and we see the same 
sight in whatever part of the world we may be. The 
Earth on which we live, is, in fact, surrounded by stars on 
all sides ; and this was so evident to even the first men 
who studied the heavens that they pictured the Earth 
standing in the centre of a hollow crystal sphere, in which 
the stars were fixed like golden nails. 

3. In the daytime the scene is changed. In place of 
thousands of stars, our eyes behold a glorious orb whose 
rays light up and warm the Earth ; and this body we call 
the su^r. So bright are its beams that, in its presence, all 
the " lesser lights," the stars, are extinguished. But, if we 
doubt their being still there, we have only to take a can- 
dle from a dark room into the sunshine to understand how 

1. What is Astronomy ? 2. Describe the appearance of the sky at tug-lit. By 
what is the Earth surrounded ? 3. In the daytime what do we behold in the sky ? 



10 INTRODUCTION. 

their feeble light, like that of the candle, is " put out " by 
the greater light of the Sun. 

4. There are, however, other bodies which attract our 
attention. The moon shines at night, now as a crescent 
and now as a full Moon, sometimes, like the Sun, render- 
ing the stars invisible. Its changes show us that there is 
some difference between it and the Sun ; for, while the Sun 
always appears round, because we receive light from all 
parts of its surface turned toward us, the shape of the bright 
portion of the Moon varies from night to night, that part 
only being visible which is turned toward the Sun. 

5. Again, if we examine the heavens more closely still, 
we may see, after a few nights' watching, one, or perhaps 
two, of the brighter " stars " change their position with 
regard to the stars lying near them, or relatively to the 
Sun if we watch that body at its rising and setting. These 
are the planets; the ancients called them "wandering 
stars." 

6. But the planets are not the only bodies which move 
across the face of the sky. Sometimes a comet may by 
its sudden appearance and strange form awaken our inter- 
est, and make us acquainted with a new class of objects, 
unlike any of those heretofore mentioned. 

7. Such are the celestial bodies ordinarily visible. Far 
away, and comparatively so dim that the naked eye can 
make little out of them, lie the nebulae (from the Latin 
nebula, "a cloud"); so called because in the telescope 
they often put on strange cloud-like forms. They differ 
as much from stars in their appearance as comets do from 
planets. 

There are other bodies, to which we shall refer by and 
by. We will here merely state in a general way what As- 



What has become of the stars ? 4. What other body do we see at night ? What 
changes does the Moon undergo, and why? 5. On a closer examination of the 
heavens, what may we see? 6. What bodies sometimes suddenly appear? 7. 
What other heavenly bodies are mentioned ? Describe the nebulae. 8 What is 



THE STAES. n 

tronomy teaches us concerning star and sun, moon and 
planet, comet and nebula. 

8. The Stars. — To begin, then, with the stars. So far 
from being stationary and fixed, as it were, in a hollow 
glass globe, at nearly equal distances from us, they are all 
in rapid motion, and their distances vary enormously; 
though all of them are so very far away that they appear 
to be at rest, as a ship does when sailing along at a great 
distance from us. In spite, however, of their great and 
varying distances, science has been able to get a mental 
bird's-eye view of all the hosts of stars which the heavens 
reveal to our eyes, as they would appear to us if we could 
plant ourselves far on the other side of the most distant 
one. The telescope — an instrument described further on — 
has, in fact, taught us that all the stars which we see form 
but a cluster of islands, as it were, in an infinite ocean of 
space. We may therefore think of all the stars which we 
see, as forming our universe ; and, when we have fixed 
that thought well in our minds, we may think of space 
being peopled with other universes, as there are other 
cities besides New York in the United States. 

9. Further, we know that our Sun is one of the stars 
that compose this cluster, and that the reason why it ap- 
pears so much larger and brighter than the rest is simply 
because it is the nearest star to us. 

We all know how small a distant house looks, or how 
feebly a distant gas-light seems to shine ; but the distant 
house may be larger than the one we are in, or the distant 
light be brighter than the one which, being nearer to us, 
renders the other insignificant. It is precisely so with the 
stars. Not only would they appear to us as bright as the 
Sun, if we were as near to them, but we know for a fact 
that some of them are larger and brighter. 

the fact with respect to the motion and distances of the stars ? What does the 
telescope teach us respecting the stars? With what may we regard space as 
peopled ? 9. What is our Sun ? Why does it appear larger and brighter than 



12 INTRODUCTION. 

i o. Now, why do the stars and the Sun shine ? They 
shine, or give out light, because they are white-hot. They 
are globes of the fiercest fire ; on their surfaces, masses of 
metals and other substances are burning together more 
fiercely than any thing we can imagine. 

u. The Planets. — What, then, are the planets? We 
may first state that they are comparatively small bodies 
travelling round our Sun at various distances from him. 
Our Earth is one of them. There is, however, an impor- 
tant difference between the planets and the Sun. We 
have seen that the Sun is white-hot ; the surface, or outer 
crust, of our Earth, on the contrary, we know to be cold- 
all the heat we get coming from the Sun — and because it 
is cold, it cannot give out light. Astronomers have 
learned that all the other planets are like the Earth in 
this respect. They are all dark bodies — having no light 
in themselves ; and they all, like us, get their light and 
heat from the Sun. When, therefore, we see a planet in 
the sky, we know that its light is sunshine second-hand ; 
that, as far as its light is concerned, it is but a looking- 
glass reflecting to us the light of the Sun. 

We have now got thus far : planets are dark or non- 
luminous bodies travelling round the Sun, which is a 
bright body — bright because it is white-hot ; and the Sun 
is a sta?% one of the stars which together form our uni- 
verse; the reason that it appears larger and brighter than 
the other stars being because it is nearer to us than they. 
It seems likely that the other stars have planets revolving 
round them, although they are so very far away that the 
telescopes we possess at present are not powerful enough 
to show us their planets, if they have any. 

12. The Moon, — We now come to the Moon. What 



the other stars ? Illustrate this. 10. Why do the stars and the Sun shine ? 11. 
What are the planets ? What important difference is there between the planets 
and the San? Whence do the planets get their light and heat? Sum up what 
we have thus far learned. Are the other stars attended by planets ? 12. What 



NEBULA AND COMETS. 13 

is it ? The Moon goes round the Earth, as the planets re- 
volve round the Sun ; it is, in fact, a planet of the Earth ; 
it is to the Earth what the Earth is to the Sun. Like the 
Earth and planets, it is a dark body, and this is the rea- 
son it does not always appear round as the Sun does. We 
only see that part of it that is lit up by the Sun. In the 
Moon we have a specimen of a third order of bodies, 
called satellites, or companions, as they accompany the 
planets in their courses round the Sun. 

We have, then, to sum up again — (1) The Sun, a star, 
like all the other stars in motion ; (2) Planets revolving 
round the Sun ; (3) Satellites revolving round the planets. 

13. Nebulae and Comets. — Nebulae and comets are very 
different from the stars and planets, for they are masses 
of gas. The nebulas lie far away from us, some of them 
perhaps out of our universe altogether. The comets rush 
for the most part from distant regions to our Sun, and 
having gone round him they go back again, and we only 
see them for a small part of their journey. 

We saw in Art. 10 that the stars shine because they 
are white-hot ; so also nebulas and comets shine because 
they are white-hot : but in the case of the stars we are 
dealing with solid or liquid matter, in the case of the neb- 
ulas and comets with burning gas. 

14. Further Facts. — Such, then, are some of the bodies 
with which the science of Astronomy has to deal; but 
astronomers have not rested content with the appearances 
of these bodies ; they have measured and weighed them, 
in order to assign to them their true place. Thus they 
have found that the Sun is 1,245,000 times larger than the 
Earth, and the Earth 50 times larger than the Moon. 
They have also discovered that, while we travel round the 



is the Moon ? Of what order of "bodies is it a specimen ? 13. How do the nehulse 
and comets differ from the stars and planets ? Why do they shine ? 14. What 
have astronomers found with respect to the comparative size of the Sun, Earth, 
and Moon ? What, with respect to the distance of the Moon and Sun from the 



U INTRODUCTION. 

Sun at a distance of 91,000,000 miles, the Moon travels 
round us at a distance comparatively insignificant — only 
240,000 miles. Thus the greater size of the Sun is bal- 
anced, so to speak, by its greater distance; the result 
being that the large distant Sun looks about the same size 
as the small near Moon. 

15. We already see how enormous are the distances 
dealt with in Astronomy, although they are measured in 
the same way as a land-surveyor measures the breadth of 
a river that he cannot cross. The numbers we obtain when 
we attempt to measure any distance beyond our own little 
planetary system convey no impression to the mind. 
Thus the nearest fixed star is more than 20,000,000,000,000 
miles away ; the more distant ones are so far away that 
their light, which travels at the rate of 185,000 miles in a 
second, requires 50,000 years to reach our eyes ! 

In spite, however, of this immensity, the methods em- 
ployed by astronomers are so sure that the distances, sizes, 
weights, and motions, of the nearer bodies, are now well 
known. We can, indeed, predict the place that the Moon 
— the most difficult one to deal with — will occupy ten 
years hence, with more accuracy than we can observe its 
position in the telescope. 

16. Here we see the utility of the science, and how 
upon one branch of it, Physical Astronomy, which treats 
of the motions and structure of the heavenly bodies, is 
founded another branch, Practical Astronomy, which 
teaches us how their movements may be made to help 
mankind. 

1 7. Usefulness of Astronomy. — Let us first see what it 
does for our sailors and travellers. A ship that leaves our 
shores for a voyage round the world takes with it a book 
called the "Nautical Almanac," prepared three or four 

Earth ? 15. What kind of distances are dealt with in Astronomy ? How far off 
is the nearest fixed star ? How far are the more distant ones ? 16. Of what does 
Physical Astronomy treat ? What branch of the science is founded on this ? 



USES OF ASTKOXOMY. 15 

years in advance by government astronomers. In this 
book, the places the Moon, Sun, stars, and planets, will 
occupy at certain stated hours for each day are given, and 
this information is all that sailors and travellers require to 
find their way across pathless seas or unknown lands. 

But we need not go on board ship or into new coun- 
tries to find out the practical uses of Astronomy. It is 
Astronomy that teaches us to measure the flow of time — 
the length of the day and the year ; without Astronomy 
to regulate them, clocks and watches would be quite use- 
less. It is Astronomy that divides the year into seasons 
for us, and teaches us the times of the rising and setting 
of the Moon, which lights up our night. It is to Astron- 
omy that we must appeal when we would inquire into the 
early history of our planet, or wish to map its surface. 

18. Such, then, is Astronomy — the science which, as its 
name, derived from two Greek words (aorrjp, a star, and 
vojiog, a law), implies, unfolds to us the laws of the stars. 

Early History of Astronomy. 

19. The establishment of the general facts just stated, 
and the various laws and principles which constitute the 
science of Astronomy at the present day, has been the 
work of centuries. The first astronomers were the Ancient 
Shepherds, who, as they tended their flocks beneath the 
canopy of heaven, naturally became interested in the orbs 
with which it was studded, observed their motions, and 
gave names to those that were most conspicuous. They 
knew, however, only such isolated facts as were apparent 
to the eye ; it was reserved for later ages to trace visible 
effects to their causes, and to build up theories ; and not 
till the improved instruments of comparatively recent 

17. Of what use is Astronomy to sailors ? Mention some of the other uses of 
Astronomy. 18. What is the meaning, and. what the derivation, of the word 
astronomy? 19. Who were the first astronomers? What facts alone were 



16 INTRODUCTION. 

times extended the field of human vision almost beyond 
belief, was it possible to penetrate the mysteries of the 
science to their depths. 

20. The Chaldeans and Egyptians were the first to 
make any material progress in Astronomy. The former, 
by continued observation, discovered that the eclipses of 
the Moon recur in the same order in periods of 18 years, 
and were thus able to predict them with considerable ac- 
curacy ; the latter investigated the motions of the planets, 
and established a sacred year of 365^ days. 

The Chinese, also, paid great attention to this science 
in very early times. More than 2,300 years before the 
Christian era (according to their own records), a tribunal 
was established for the prosecution of astronomical studies, 
and particularly for the prediction of eclipses. Its mem- 
bers were held responsible with their lives for the correct- 
ness of their calculations ; and we are told that one of the 
emperors actually put to death his two chief astronomers 
for failing to predict an eclipse of the Sun. 

21. From Egypt, the cradle of learning, art, and sci- 
ence, the Greeks obtained their first knowledge of astron- 
omy, to which their wise men made important additions. 
Thales, about 600 b. c, taught that the world was round, 
and that the Moon shone with reflected light. His pupil 
Anaximander conceived the bold idea of a plurality of 
worlds — that is, that the planets are inhabited. A little 
later, Pythag'oras is said to have advanced the opinion 
that the Earth and other planets revolve round the Sun. 
Whether he did so or not, it is certain that this was taught 
by Aristarchus about 280 b. c. ; as, also, that the distance 
of the Sun from the Earth is insignificant in comparison 

known to them ? 20. What nations were the first to make any material progress 
in astronomy ? What did the Chaldeans discover ? What did the Egyptians in- 
vestigate? What evidence is there that the Chinese paid great attention to as- 
tronomy in early times ? 21. Whence did the Greeks obtain their knowledge 
of astronomy ? What was taught by Thales ? What, by Anaximander ? What, 
by Pythagoras? What, by Aristarchus? What was done by Eratosthenes? 



EAELY HISTOEY OF ASTEONOMY. J7 

with that of the stars. Among other famous Greek astron- 
omers were Eratos'thenes, who devised an accurate method 
of measuring the circumference of the Earth, and Hippar- 
chus, who made a catalogue of all the stars visible above 
his horizon. 

Ptolemy, an eminent Egyptian astronomer who flour- 
ished in the second century after Christ, rejected the the- 
ory of Pythagoras and Aristarchus respecting the solar 
system, and advanced one of his own, which soon met with 
general acceptance. He taught that the Earth was the 
centre of a system of eight immense hollow spheres of crys- 
tal, placed one within another : that the Moon was in the 
nearest sphere ; Mercury in the next ; Yenus in the third ; 
the Sun in the fourth ; Mars, Jupiter, and Saturn, in the 
fifth, sixth, and seventh, respectively ; and that the eighth 
belonged to the stars, which, though most distant, were 
still visible through the transparent crystal. The revolu- 
tion of this cumbrous system round the Earth from east to 
west, once in twenty-four hours, he thought would ac- 
count for the succession of day and night, and the various 
phenomena of the heavens. 

22. During the Dark Ages, Astronomy was cultivated 
chiefly by the Arabians, who made no advance as regards 
theory, but were diligent observers, and devised some im- 
provements in instruments and methods of calculation. 
Even after the termination of this period, comparatively 
little progress was made until the time of Coper'nicus, a 
German priest, about 350 years ago. He ventured to 
reject the system of Ptolemy, which then generally pre- 
vailed; and, reviving the teachings of Pythagoras and 
Aristarchus, set forth what is called from him the Coper- 
nican system, now very generally received as true, though 
at first bitterly denounced as visionary and even irreli- 

What, by Hipparchus ? What theory was advanced by Ptolemy ? 22. By whom 
was astronomy chiefly cultivated during the Dark Ages ? What improvements 
were made by the Arabians ? When and by whom was the present theory of the 



18 INTRODUCTION. 

gious. Its three fundamental points are, (1) that the 
Earth is round ; (2) that it turns on its axis from west to 
east; and (3) that the Earth and other planets revolve 
round the Sun. 

23. After Copernicus came the great Italian philoso- 
pher Galileo, who first used the telescope, and was thus 
enabled to make many important discoveries, all tending 
to support the theory of Copernicus. The day on which 
Galileo died was memorable for the birth of Newton, 
whose great discovery of the law of gravitation explained 
the planetary motions, while his mathematical researches 
gave a new impetus to the science. 

Mathematical Definitions. 

Certain mathematical terms used in Astronomy must 
be understood. 

24. A Line is a magnitude conceived as having length 
without breadth or thickness. 

25. A Straight Line is one that has the same direction 
Fig. 1. throughout. It is the shortest distance 

A ' B 

= between two points. A B and CD are 

C D . , ,. 

fig. 2. straight lines. 

A Curved Line, or Curve, is a line 
that changes its direction at every point, 
L F ^EF. 

Parallel Lines are such as maintain the same distance 
from each other at all points, as A B and CD in Figure 1. 

26. An Angle is the difference in direction of two 
straight lines that meet. The point at which they meet 
is called the Vertex of the angle. 

An angle is named from the letter at its vertex, if but 



universe advanced ? What are its three fundamental points ? 23. By what two 
philosophers was Copernicus succeeded, and what discoveries did they make ? 
24. What is a Line ? 25. What is a Straight Line ? What is a Curved Line ? 
26. What is an Angle ? How is an angle named ? 27. When are two lines said 




MATHEMATICAL DEFINITIONS. 19 

one angle is formed there. Otherwise, it is named from the 
letters on each side and at the vertex, that at the vertex 
fig. 3. being placed in the middle. The angle in 

Fig. 3 is called K ' ; if more than one an- 
gle were formed there, it wonld be dis- 
tinguished as IKL or L KI. 
The size of an angle depends not at all on the length 
of its sides, but simply on their difference of .direction. 
The angle iTwill become no larger, however far we may 
extend its sides. 

Fig. 4. 27. When one straight line meets 

N another in such a way as to make the 

two adjacent angles equal, the lines are 
said to be perpendicular to each other, and 




p ° Q the angles formed are called Right Angles. 
The angles NOP and NO Q, being equal, are right 
angles ; and the lines NO, P§, are perpendicular to 
each other. 

Fig. 5. An Obtuse Angle is one that is 

greater than a right angle, as M S T. 
An Acute Angle is one that is less than 
~s v a right angle, as M 8 V. 

28. A Surface is a magnitude conceived as having 
length and breadth without thickness. 

A Plane is a surface with which a straight line that 
joins any two of its points will coincide altogether. 

A Convex Surface is one that swells out in a rounded 
form, as the outside of an egg-shell. 

A Concave Surface is one that curves in, as the inside 
of an egg-shell. 

29. A Plane Figure is a plane bounded by a line or 
lines. 



to be perpendicular to each other ? What are the angles formed by lines that are 
perpendicular to each other called? What is an Obtuse Angle? What is an 
Acute Angle? 28. What is a Surface? What is a Plane? What is a Convex 
Surface ? What is a Concave Surface ? 29. What is a Plane Figure ? 30. What 



20 



INTRODUCTION. 



30. A ^Triangle 
bounded by three 
XYZ. 

A Circle is a plane figure bounded 



is a plane figure 
straight lines, as 




s-i- 



Fig. 7. 




V z 

by a curve, every point of which is 
equally distant from a point within, 
called the Centre. 

The Circumference of a circle is the 
line that bounds it. Any part of the 
circumference is called an Arc. Fig. 1 
represents a circle; A is the centre, 
B E D the circumference ; E B, B D, D E, etc., are 
arcs. 

A Diameter of a circle is a straight line passing 
through its centre, and terminating at each end in the 
circumference, as BE in Fig. 7. Every circle has an 
infinite number of diameters, all equal. 

A Radius of a circle is a straight line drawn from the 
centre to the circumference ; as A B, A IT, A 6r, A E, in 
Fig. 1. Every circle has an infinite number of radii, all 
equal, and each just half its diameter. 

Fig. 8. A Tangent is a straight line that 

touches the circumference of a circle 
in a single point, without cutting it at 
either end when produced ; as A B in 
Fig. 8. 

32. The circumference of every 
circle may be divided into 360 equal 
parts, called Degrees (marked °). Each degree may be 
divided into 60 equal parts, called Minutes (') ; and each 
minute into 60 equal parts, called Seconds ("). 

A circumference may also be divided into 12 equal 
parts, of 30 degrees each. These are called Signs. 

is a Triangle ? 31. What is a Circle ? What is the Circumference of a circle ? 
What is an Arc ? What is a Diameter ? What is a Radius ? What is a Tangent ? 
32. Into what may the circumference of every circle be divided ? What is a Sign ? 




MATHEMATICAL DEFINITIONS. 



21 



33. A Semicircle is one-half, a Quadrant one-fourth, 
and a Sextant one-sixth, of a circle. 

34. An angle is measured by the number of degrees in 
the arc that subtends it. A right angle is subtended by 
one-fourth of the circumference (see C D E in Fig. 8), 
and is therefore an angle of 90 degrees. 

Fig. 9. • 35. An Ellipse is a curve 

every point of which is at such 
distances from two points with- 
in, called its foci, that the sum 
of these distances is in each case 
the same. AJB G is an ellipse; 
D and jB'are its foci. A D + A E 
- BJD +BE= CD + CM 

An ellipse may be described by fastening the ends of a piece of thread 
at any two points (the -foci) whose distance from each other is less than 
the length of the thread, and then drawing a line around these points 
with a pencil placed against the thread, and kept stretched out as far as 
the thread will allow. The process is shown in Fig. 10. 





Mode of describing an Ellipse. 



The thread here represents the sum of the distances from each point 
of the ellipse to the foci, and remains the same while the ellipse is 
described. Draw an ellipse according to these directions. 

The Centre of an ellipse is the point midway between 
the foci in the straight line that connects them, as O in 
Fig. 10. A Diameter is a line passing through the centre 
and terminating at each end in the ellipse ; as G H, I J. 

33. What is a Semicircle ? What is a Quadrant ? What is a Sextant? 34. How 
is an angle measured ? Illustrate this in the case of a right angle. 35. What is 
an Ellipse ? How may an ellipse be drawn ? What is the Centre of an ellipse ? 



22 



INTRODUCTION. 




The Major Axis of an el- 
lipse is its longest diameter, as 
GH. The Minor Axis is its 
shortest diameter, as IJ. 

The Eccentricity of an el- 
lipse is the distance of either 
focus from the centre, divided 
by half the major axis. Hence 
the greater the aforesaid dis- 
tance, the greater the eccentricity, and the more the ellipse 
deviates from a circle. In Fig. 11, if D F, the distance 
of one of the foci from the centre, is to G- F, half the 
major axis, as 2 to 3, the eccentricity of the ellipse will 
be |, or .66+. 

36. A Solid is a magnitude that has length, breadth, 
and thickness. 

37. A Sphere is a solid bounded by a curved surface 
every point of which is equally distant from a point 

within, called the centre. A 
Hemisphere is half a sphere. 

A Diameter of a sphere is a 
straight line passing through its 
centre, and terminating at each 
end in its surface. 

The Axis of a revolving 
sphere is the diameter round 
which it turns. The Poles are 
the extremities of the axis. In 
the sphere represented in Fig. 
12, the straight line connecting A and B is a diameter; 
and also the axis, if the sphere revolves round this 
diameter ; in which case A and B are the poles. 




What is a Diameter ? What is the Major Axis ? The Minor Axis ? What is 
meant by the Eccentricity of an ellipse ? 36. What is a Solid ? 37. What is a 
Sphere ? What is a Hemisphere ? What is a Diameter of a sphere ? What is 
the Axis of a revolving sphere ? What are the Poles ? 38. What is a Great 



MATHEMATICAL DEFINITIONS. 23 

38. Circles on the surface of a sphere are distinguished 
as Great and Small. A Great Circle is one whose plane 
divides the sphere into two equal parts ; as A SB G and 
G IT, in Fig. 12. A Small Circle is one whose plane 

Fig. 13. divides the sphere into two unequal parts, 

Oas CD and EF. 
The Circumference of a sphere is one 
of its great circles. The Equator of a 
sphere is that great circle which is equal- 
ly distant from the two poles, as G H in 
Fig. 12. 

39. A Spheroid is a solid which differs Fig. 14. 
but little from a sphere. 4 

An Oblate Spheroid is a sphere flat- 
tened at the poles, as in Fig. 13. 

A Prolate Spheroid is a sphere length- 
ened out at the poles, as in Fig. 14. 




CHAPTER I. 

THE STARS. 

Magnitudes and Distances of the Stars. 

40. Magnitudes of the Stars. — The first thing that 
strikes us when we look at the stars is, that they vary 
very much in brightness. All of those visible to the 
naked eye are divided into six classes of brightness, called 
Magnitudes, so that we speak of a very brilliant one as " a 



Circle ? What is a Small Circle ? What is the Circumference of a Sphere ? The 
Equator of a Sphere ? 39. What is a Spheroid ? An Ohlate Spheroid? A Pro- 
late Spheroid ? 

40. What is the first thing that strikes us when we look at the stars ? As re- 



24 THE STAES. 

star of the first magnitude : " of the feeblest visible, as " a 
star of the sixth magnitude," and so on. 

The number of stars of all magnitudes visible to the 
naked eye under the most favorable circumstances, is 
about 6,000 ; so that the greatest number visible at any one 
time — as we can only see half of the sky at once — is 3,000. 
If we use a small telescope this number is largely in- 
creased, as that instrument enables us to see stars too 
feeble to be perceived by the eye alone. The stars thus 
revealed to us are called Telescopic Stars. These also vary 
in brightness ; and the classification is continued down to 
the twelfth, fourteenth, sixteenth, and even higher mag- 
nitudes, according to the power of the telescope. With 
powerful telescopes, at least 20,000,000 stars down to the 
fourteenth magnitude are visible. 

41. Comparative Brightness. — A star of the 6th magni- 
tude is, as we have seen, the faintest visible to the naked 
eye. Taking the average brightness of a 6th-magnitude 
star as unity, the average brightness of the other classes 
is estimated as follows : — 



6th magnitude, 1 

5th " 2 

4th " 6 



3d magnitude, 12 

2d " 25 

1st " 100 



Sirius, the brightest star of the 1st magnitude, 324 

The Sun, the nearest star to us, 6,480,000,000,000 

Even stars of the same magnitude differ considerably 
in brilliancy. It will be seen from the above table that 
the brightness of Sirius is more than three times as great 
as the average brightness of its class. 

42. The stars are usually divided about as follows : of 
the first magnitude, 20; of the second, 65; of the third, 

gards brightness, how are the stars visible to the naked eye divided ? How many 
are there ? How many can be seen at once ? What are Telescopic Stars ? How 
are they divided ? How many stars are there, including those of the 14th magni- 
tude? 41. What is the comparative brightness of the stars of the first six mag- 
nitudes ? How does the brightness of Sirius compare with the average brightness 
of its class ? How does the Sun compare in brightness with a star of the 6th 
magnitude ? 42. State the number of stars of each of the first six magnitudes. 



DISTANCES OF THE STAKS. 25 

200; of the fourth, 450; of the fifth, 1,100; of the sixth, 
about 4,000. The number increases largely as we descend 
in the scale of brilliancy. 

43. Distances of the Stars. — It is evident that the stars, 
as they shine with such different lights, one star differing 
from another star in glory, are either of the same size at 
very different distances, the most remote being of course 
the- faintest; or are of different sizes at the same distance, 
the largest shining the brightest ; or are of different sizes 
at different distances. In the case of twelve stars the 
actual distances are known, and differ greatly ; as regards 
the rest, we can only say it is most probable that the 
difference in brilliancy is due mainly to difference of 
distance. 

44. The distances of the stars from us are so great 
that to state them in miles hardly gives an adequate idea 
of them ; some other method, therefore, must be used, and 
the velocity of light affords us a convenient one. 

Light travels at the rate of 185,000 miles in a second — 
that is to say, between the beats of the pendulum of an 
ordinary clock, light travels a distance equal to eight 
times round the Earth. Now, the nearest star (leaving 
the Sun out of the question) is situated at a distance which 
light, even with the extraordinary velocity just mentioned, 
requires three and a half years to traverse. We may say 
that, on an average, light requires fifteen and a half years 
to reach us from a star of the 1st magnitude, twenty-eight 
years from a star of the 2d, forty-three years from a star 
of the 3d, one hundred and twenty from a star of the 6th, 
and so on, until for stars of the 12th magnitude the time 
required is thirty-five hundred years. If, therefore, a 
star of the 6th magnitude were destroyed at the pres- 
ent moment, we should continue to see it in the heavens 
for 120 years to come; and if one of the 12th magnitude 

43. To what are the different degrees of brightness in the stars due? 44. Give 
an idea of the distances of the stars, as measured by the velocity of light. 

2 



26 THE STAES. 

were now created, it would be 3,500 years before it would 
be perceptible to us. 

45. The Diameters of the Stars cannot be determined by 
our most powerful instruments. As seen from the Earth, 
they are, in consequence of their distance, mere points of 
light, so small as to be beyond our most delicate measure- 
ment. The Moon, which travels very slowly across the 
sky, sometimes gets before, or eclipses, or occults, some of 
them ; but they vanish in a moment — which they would 
not do, if they were not as small as we have stated. 

46. The Milky Way.— Winding among the stars, a 
beautiful belt of pale light spans the sky, and sometimes 
it is so situated as to divide the heavens into two nearly 
equal portions. This belt is the Milky Way (see Celestial 
Charts at the end of the volume) ; and the smallest tele- 
scope shows that it is composed of stars so faint, and 
apparently so near together, that the eye can perceive 
only a dim continuous glimmer. Milton alludes to it 
as the 

" Broad and ample road 
Whose dust is gold, and pavement stars." 

Among the Greeks, the Milky Way was known as the Galaxy and the 
Circle of Milk. The Chinese and Arabians call it the Celestial Eiver. 
Some of the American Indian tribes regarded it as the path of departed 
souls to the spirit-land. In England it used to be familiarly called 
Jacob's Ladder. 

Different opinions prevailed among the ancients as to what it was. 
Aristotle thought it was the result of gaseous exhalations from the earth 
set on fire in the sky. Theophrastus believed it to be the soldering 
together of two hemispheres constituting the celestial vault. Diodo'rus 
represented it as a dense celestial fire, appearing through the clefts of 
parting hemispheres. Democ'ritus and Pythagoras divmed the truth, 
that the Galaxy is nothing more or less than a vast assemblage of very 
distant stars ; and Ovid speaks of it as a highway whose groundwork is 
of stars. 

45. What is said of the size or diameters of the stars ? What happens when the 
Moon eclipses one of them? 46. What is the Milky Way? In what terms does 
Milton allude to it ? What did the Greeks call the Milky Way ? The Chinese 
and Arabians ? How did some of the American Indians regard it ? What did it 
use to be called in England ? Give some of the views of the ancients respecting 



THE MAGELLANIC CLOUDS. 27 

47. The Magellanic Clouds, called the Nubec'ula Major 
and Nubec'ula Minor, distinctly visible in the southern 
hemisphere on a clear moonless night, are two cloudy oval 
masses of light, very like portions of the Milky Way, but 
apparently unconnected with its general structure. The 
telescope resolves them into single stars, star-clusters, and 
nebulous matter in various degrees of condensation. They 
are named from the Portuguese navigator Magellan, though 
he was not the first to observe them. 

Fig. 15 shows the appearance which part of the Nubec- 
ula Major presents, when viewed through the telescope. 



H BsBi 




j^HS I BB M > 


■ffiBP^i* * . ?>;>"%V -fl **•: 


.•:.', '- ■ ',_ ■. - ' , . A 




■ 





Fig. 15.— Part of the Nubecula Major. 

Shape of our Universe. 

48. Distribution of the Stars. — The largest stars are 
scattered very irregularly ; but, if we look at the smaller 
ones, we find that they gradually increase in number as 
they approach the Milky Way. In fact, of the 20,000,000 
stars visible, as we have stated, in powerful telescopes, at 
least 18,000,000 lie in and near the Milky Way. 

the Milky Way. 47. Describe the Magellanic Clouds. Into what does the tele- 
scope resolve them ? 48. How do we find the largest stars distributed ? How, 



28 



THE STAES. 



49. Adding this fact to what has been said about the 
distances of the stars, we can now determine the shape of 
our universe. It is clear that it is most extended where 
the faintest stars are visible, and where they appear near- 
est together ; because they appear faint in consequence of 
their distance, and because their apparent close packing 
arises, not from actual nearness to each other, but from 
their lying in that direction at constantly increasing dis- 
tances. Indeed, the stars which form the Milky Way, 
extending one behind another to an almost infinite dis- 
tance, are probably as far from each other as our Sun is 
from the nearest star. 

50. The Milky Way, then, traces for us the direction 
in which our universe has its largest dimensions. The ab- 
sence of faint stars in the parts of the sky farthest from 
the Milky Way shows us that the limits of the universe 

in that direction 
are much sooner 
reached than in 
the direction of 
the Milky Way 
itself. We 
gather, there- 
fore, that the 
thickness of our 
universe is small 
compared with 
its length and 
breadth ; that its 
shape is not 
spherical, but 
rather that of a 
as the Milky 




Fig. 



16.— Supposed Shape of our Universe 

circular piece of thick pasteboard. And 



the smaller ones ? 49. What may be inferred respecting the shape of our uni- 
verse ? 50. What does the Milky Way trace for us ? What does the absence of 
faint stars in the parts of the sky remote from the Milky Way show ? What is 



SHAPE OF OUR UNIVERSE. 29 

Way divides into two principal branches, which, after pur- 
suing separate courses nearly half its length, again unite, 
we infer that this flattened stratum of stars is divided 
lengthwise, as if the rim of the pasteboard were split and 
its two surfaces pulled apart at a small angle through 
half the circle, as in Fig. 16. 

To account for the appearances presented, we must re- 
gard our solar system as lying nearly at the centre of this 
mass of stars, and near the region at which it begins to 
divide ; but, as there are more stars on the south side of 
the Milky Way than there are on the north, we gather 
that our Earth occupies a position somewhat to the north 
of the middle of its thickness. 

On this supposition, all the stars which, owing to our 
position in observing them, appear so remote from the 
Milky Way, really form part of it, and our great Sun rep- 
resents but an atom of its luminous sand. 

5 1 . Although the Milky Way thus enables us to get a 
rough idea of the shape of our universe, as we get an idea 
of the shape of a wood from some point within it by see- 
ing in which direction the trees appear thickest together, 
still the telescope teaches us that its boundaries are prob- 
ably very irregular. 

The Constellations. 

52. We have thus far considered our star-system as a 
whole, its dimensions, and shape. Before we proceed with 
a detailed examination of the stars composing it, it will be 
convenient to state the groupings into which they have 
been arranged, and the way in which any particular star 
may be referred to. 

53. The stars, then, from remote antiquity, have been 
classified into groups called Constellations, each constella- 

inferred from the fact that the Milky Way divides into two principal branches ? 
Where must we regard our solar system as lying? 51. What does the telescope 
teach us respecting the boundaries of the Milky Way ? 53. How have the stars 



30 



THE STABS. 



tion being fancifully named after some object (in most 
cases, an animal or mythological personage) which the ar- 
rangement of the stars composing it was thought to sug- 
gest. For the most part, however, little or no resemblance 
can be traced to the object after which the group is 
named; compare, for instance, the outline of the Great 
Bear with the position of the principal stars composing 
that constellation, as shown in Fig. 17. 




Fig. 17.— Constellation or the Great Beak. 

The resemblance being in most cases so remote that the effort to trace 
the figures in the sky is unsatisfactory and confusing, it has been thought 
better to present, in the Celestial Charts at the close of this volume, the 
boundaries and relative positions of the constellations, as indicated by 
dotted lines, than to delineate the animals and fabulous personages from 
which they are named. 

54. Some of the most marked constellations probably 
received their names 1,500 years before the Christian era, 
and all the leading ones were known in the time of Ara'~ 

from remote antiquity been classified ? After what are the constellations named ? 
54. How early were the leading constellations known ? Who added two in the 



THE CONSTELLATIONS. 31 

tus (270 b. a), who described them in verse. About 150 
years after Christ, Ptolemy arranged in 48 constellations 
the 1,022 stars which Hipparchus had observed at Rhodes 
250 years before. Tycho Brahe, a Danish astronomer, 
added two constellations in the sixteenth century ; and to 
these 50 (called the ancient constellations), 59 have since 
been added, making the present number 109. 

55. The Zodiacal Constellations. — The Latin and 
English names of the ancient constellations and the most 
important modern ones will now be given. We begin 
with the twelve through which the Sun passes in his 
annual round. These are called the Zodi'acal Constella- 
tions; they are to be very carefully distinguished from 
the signs of the zo'diac bearing the same name. Their 
names should be learned in the order in which they are 
presented, and should be found on the Celestial Charts at 
the end of the volume, where the constellations are laid 
down in order on the heavy circle called the Ecliptic : — 

Li'bra, the Balance. 

Scorpio, the Scorpion. 

Sagitta'rius, the Archer. 
Capricornus, the Goat. 
Aquarius, the Water-bearer. 
Pisces, the Fishes. 

The order of the names may perhaps be more readily 
remembered, if they are thrown together in rhyme : — 
The Earn and Bull lead off the line ; 
Next Twins, and Crab, and Lion, shine, 

The Virgin and the Scales ; 
Scorpion and Archer next are due, 
The Goat and Water-bearer too, 

And Fish with glittering tails. 

56. The Northern Constellations are those which are 

16th century ? How many have been added in modern times ? What is the pres- 
ent number? 55. What is meant by the Zodiacal Constellations? From what 



A'ries, 


the Ram. 


Taurus, 


the Bull 


Gem'ini, 


the Twins. 


Cancer, 


the Crab. 


Leo, 


the Lion. 


Virgo, 


the Virgin. 



32 



THE STARS. 



visible above the zodiacal constellations. The principal 
ones are as follows (see Celestial Chart of the Northern 
Hemisphere) : — 



Ursa Major, 
Ursa Minor, 
Draco, 
Cepheus, 
Boo'tes, 

Goro'na Borea'lis, 
Hercules, 
Lyra, 
Cygnus, 
Cassiopeia, 
Perseus, 
Auri'ga, 
Ophiuchus, 
Serpens, 
Sagitta, 
A'quila, 
Delphi'nus, 
Equuleus, 
Peg'asus, 
Androm'eda, 
Triangulum, 
Camelopar dolus, 
Canes Venat'ici, 
Coma Berenices, 
Vulpec'ula et Anser, 
Cor Car' oli, 



the Great Bear. 

the Little Bear. 

the Dragon. 

Cepheus. 

Bootes. 

the Northern Crown. 

Hercnles. 

the Lyre. 

the Swan. 

Cassiopea (the Lady's Chair). 

Perseus. 

the Wagoner. 

the Serpent-bearer. 

the Serpent. 

the Arrow. 

the Eagle. 

the Dolphin. 

the Little Horse. 

the Winged Horse. 

Andromeda. 

the Triangle. 

the Camelopard. 

the Hunting Dogs. 

Berenice's Hair. 

the Fox and the Goose. 

Charles's Heart. 



57. The Southern Constellations. — The principal con- 
stellations visible in the United States below the zodiacal 
ones, called Southern Constellations, are as follows (see 
Celestial Chart of the Southern Hemisphere) : — 



are they to be distinguished? Name the Zodiacal Constellations in order. 56. 
Name some of the principal Northern Constellations. 57. Name some Southern 



SOUTHERN CONSTELLATIONS. 33 

Cetus, the Whale. 

Ori'on, Orion. 

JEJrid'anics, the River Eridanus. 

Lepus, the Hare. 

Ganis Major, the Great Dog. 

Canis Minor, the Little Dog. 

Argo Ndvis, the Ship Argo. 

Hydra, the Snake. 

Crater, the Cup. 

Corvus, the Crow. 

Centauries, the Centaur. 

Zupus, the Wolf. 

Coro'na Austra'lis, the Southern Crown. 

Piscis Australis, the Southern Fish. 

Monoc'eros, the Unicorn. 

Columba JYbachi, Noah's Dove. 

58. Names of the Stars. — The whole heavens being 
portioned out among these constellations, the next thing 
to be done was to invent some method of referring to 
each particular star. The method introduced by John 
Bayer, of Augsburg, in 1603, and now in use, is to arrange 
all the stars in each constellation in the order of bright- 
ness, and to attach to them in that order the letters of the 
Greek alphabet, using after the letters the genitive of the 
Latin name of the constellation. Thus, Alpha (a) Jjyraz 
denotes the brightest star in the Lyre; Beta ((3) JJrsm 
Minoris, the next to the brightest star in the Little Bear. 

Except, however, in the case of the brighter stars of a 
constellation, this alphabetical arrangement has not been 
strictly adhered to, and consequently it does not always 
indicate the relative brilliancy of the less important stars. 

59. The letters of the Greek alphabet, and their names, 
are as follows : — 



Constellations visible in the United States. 58. Describe and illustrate the 
method of referring to particular stars. 59. Repeat the Greek alphabet. After 



34 



THE STABS. 



Alpha. 

Beta. 

Gamma. 

Delta. 

Epsilon. 

Zeta. 

Eta. 

Theta. 



t, Iota, 

/c, Kappa. 

\ Lambda. 

//, Mu. 

v, Nu. 

i, xi. 



Omicron. 
Pi. 



p, Kho. 

cr, Sigma, 

r, Tau. 

v, Upsilon. 

0, Phi. 

X, Chi. 

V, Psi. 

w, Omega. 



After the Greek alphabet is exhausted, the Roman 
alphabet is used in the same way ; and after that recourse 
is had to numbers. 

6o. Some of the brightest stars are still called by the 
Arabian or other names by which they were formerly 
known. Thus, a Cards Majoris is known also as Sir'ius ; 
a Bootis, as Arctu'rus ; j3 Orionis, as Rigel ; a Lyrce, as 
Vega ; a Tauri, as Aldeb'aran ; a Ursce Minor is, as 
Pola'ris (the Pole-star), etc. 

6i. The constellations that have been named, and 
their principal stars, can be seen on the charts at the end 
of this book, or on a Celestial Globe. Before proceeding 
further, the student should make himself familiar with 
them, that he may know their relative positions when he 
comes to trace them in the sky. 

In star-maps the stars are laid down as we actually see 
them in the heavens, looking at them from the Earth ; but 
in globes their positions are reversed, as the Earth, on 
which the spectator is placed, is supposed to occupy the 
centre of the globe, while we really look at the globe from 
the outside. If, therefore, the brighter of two stars ap- 
pears on the right of the other in the heavens, it will be 
shown on its right in a star-map, but to the left of it on a 
globe. 

62. Stars of the First Magnitude. — The twenty brightest 
stars in the heavens, or first-magnitude stars, are as fol- 

the Greek alphabet is exhausted, what are used in naming the stars ? 60. What 
other names have some of the brightest stars ? Give examples. 61. How are 
the stars laid down in star-maps ? How, on globes ? Name the twenty brightest 



MOTIONS OF THE STAKS. 



35 



lows; they are given in the order of brightness, and 
should be found on the charts or on a globe : — 



Sirius, 


in Canis Major. 


Aldeb'aran, 


in Taurus. 


Cano'pus, 


" Argo Navis. 


Beta, 


" Centaurus. 


Alpha, 


" Centaurus. 


Alpha, 


" Crux. 


Arcturus, 


" Bootes. 


Anta'res, 


" Scorpio. 


RigeL 


" Orion. 


Altair, 


" Aquila. 


Capella, 


" Auriga. 


Spica, 


" Virgo. 


Vega, 


" Lyra. 


Fomalhaut, 


" Piscis Australis 


Pro'cyon, 


" Canis Minor. 


Beta, 


" Crux. 


Betelgeuse, 


" Orion. 


Pollux, 


" Gemini. 


Achernar, 


" Eridanus. 


Kegulus, 


« Leo. 



Motions of the Stars. 

63. Proper Motion. — Now, although the stars and con- 
stellations retain the same relative positions as they did 
in ancient times, all the stars are, nevertheless, in motion ; 
and in some of those nearest to us, this motion, called 
Proper Motion, is very apparent, and has been measured. 
Thus Arcturus is travelling at the rate of at least fifty- 
four miles a second, or three times faster than our Earth 
travels round the Sun — or six thousand times faster than 
an ordinary railway train. 

64. Nor is our Sun, which be it remembered is a star, 
an exception ; it is approaching the constellation Hercules 
at the rate of four miles in a second, carrying its system 
of planets, including our Earth, with it. Here, then, we 
have an additional cause for a gradual change in the posi- 
tions of the stars, for a reason we shall readily understand, 
if, when we walk along a gas-lit street, we notice the dis- 
tant lights. We shall find that the lights we leave behind 
close up, and those in front of us open out as we approach 
them : so the stars which our system is approaching are 



stars, and state what constellation each is in. 63. What is meant by the Proper 
Motion of the stars ? At what rate is Arcturus travelling ? 64. What additional 
cause have we for a gradual change in the positions of the stars ? How is this 
illustrated in the case of a gas-lit street ? 65. From what motions are the Proper 



36 



THE STAES. 



slowly opening out, while those we are quitting are closing 
up, as our distance from them increases. 

65. Apparent Motion. — The real motions of the stars — 
called, as we have seen, their proper motions — and the 
one we have just pointed out, are, however, to be gathered 
only from the most careful observation, made with the 
most accurate instruments. There are Apparent Motions, 
which may be detected in half an hour by the most care- 
less observer. These are caused, as we shall fully explain 
in Chap. XII., by the two real motions of the Earth, first 
round its own axis, and secondly round the Sun. 



Double and Multiple Stars. 

66. An examination of the stars with a powerful tele- 
scope, reveals to us the most startling and beautiful 
appearances. Stars which appear 
single to the unassisted eye, appear 
double, triple, and quadruple ; and 
in some instances the number of 
stars revolving round a common 
centre is even greater. Because 
our Sun is an isolated star, and be- 
Fig. 18-Okbit of a double cause the planets are now dark 
Stak - bodies, instead of shining, like the 

Sun, by their own light, as they once must have done, it 
is difficult, at first, to realize such phenomena, but they 
are among the most firmly-established facts of modern 
astronomy. 

A beautiful star in the constellation Lyra will at once 
give an idea of such a system, and of the use of the tele- 
scope in these inquiries. The star in question, Epsilon (e) 
Lyroe, to the naked eye appears as a faint single star. A 




Motions to be distinguished ? By what are the Apparent Motions caused ? 66. 
What changes in the appearance of some stars does a powerful telescope pro- 
duce ? Give an account of Epsilon Lyrce. In what times will the members of this 



DOUBLE AND MULTIPLE STAES. 



37 




small telescope, or opera-glass even, suffices to show it 
double, and a powerful instrument reveals the fact that 
each star composing this double is itself double ; hence it 
is known as " the Double-double." Here, then, we have 
a system of four stars; the stars composing each pair, 
considered by themselves, revolving round a point be- 
tween them ; while the two 
pairs, considered as two single 
stars, perform a much larger 
journey round a point situated 
between them. 

It may be stated roundly 
that the wider pair will com- 
plete a revolution in 2,000 
years ; the closer one, in half 
that time ; and possibly both 
double systems may revolve 
round the point lying between 
them in something less than a 
million of years. 

6j. Of the multiple stars, 
that is, such as are resolved 
by the telescope into more 
than four single stars, Theta 
(6) Orionis is one of the most 
interesting. What appears 
to the unaided eye as a single 
luminous point, is shown by 
a powerful telescope to con- 
sist of seven stars, arranged as shown in Fig. 20. 

68. More than 6,000 double stars are now known, in 
nearly 700 of which a regular orbital motion, and in some 
of them a very rapid motion, has already been detected. 



Fig. 19. —The Double-double 
Star in the Constellation 
Lyka. 1. As seen in an opera- 
glass. 2. As seen in a small tele- 
scope. 3. As seen in a telescope 
of great power. 




Fig. 20. 



-The Multiple Stab, 
Orionis. 



system complete their revolution ? 67. Describe Theta Orionis, as seen through 
a powerful telescope. 68. How many double stars are now known ? In how 
many of these has an orbital motion been detected ? How do the double stars 



38 THE STAES. 

In some cases the brilliancy of the component stars 
is nearly equal ; in others the light is very unequal. For 
instance, a first-magnitude star may have a companion 
of the fourteenth magnitude. Sirius has at least one such 
companion. Here is a list of some double stars, showing 
the time in which a complete revolution is effected : — 



Years. 

Zeta (£) Her cutis, . . 36 
Eta {r\) Corotice Borealis, 43 
Zeta (£) Cancri, ... 60 
Alpha (a) Centauri, . 15 
Omega (w) Leonis, . . 82 



Years. 

Gamma (y) Coronw 

Borealis, . . . . 100 

Delta (d) Cygni, . . 178 

Beta (/3) Cygni, . . 500 

Gamma (y) Leonis, . 1200 



6g. In the case, then, of nearly 700 double stars, in 
which an orbital motion of the component stars round a 
common centre of gravity has been observed, there can 
be no doubt that we have connected systems. Pairs thus 
connected are called Binary Stars, or Physical Couples, to 
distinguish them from unconnected double stars, or Opti- 
cal Couples, in which the component stars are really dis- 
tant from each other, their apparent nearness being due 
to their lying in the same straight line as seen from the 
Earth. 

70. When the distance of a binary star is known, we 
can determine the dimensions of the orbit of one star round 
the other, as we can determine the Earth's orbit round the 
Sun. Thus, in the case of the binary star 61 Cygni, its dis- 
tance from us being known, it is found that the orbit of the 
smaller of the two stars has a mean radius of about 45 
times the distance of the Earth from the Sun, or more 
than 4,275,000,000 miles. And yet so immense is the dis- 
tance of the two stars from us, that to the naked eye they 
seem as one. 

differ, as regards the comparative brightness of the individual stars composing 
them ? Mention some of the double stars, with their period of revolution, and 
point to them on the Celestial Chart. 69. What are Binary Stars, or Physical Cou- 
ples ? From what must they be distinguished ? 70. When the distance of a 
binary star is known, what can be determined ? What is the size of the orbit 



VARIABLE STARS. 39 

Variable Stars. 

71. Variations in Brightness. — The stars are not only 
of different magnitudes (Art. 40), but the brilliancy of 
some particular stars changes from time to time. Stars 
whose brightness varies slowly, regularly, and within cer- 
tain limits, are called Variable Stars, or briefly Variables. 
In some few cases, however, the increase and decrease 
have been sudden, and in others the limits of change are 
unknown; hence we read of new stars, lost stars, and 
temporary stars, in addition to the more regular variables. 
There is little doubt, however, that all these phenomena 
are the same in kind, though different in degree. 

72. Amount and Period of Variation. — The variation 
in brightness is measured by the difference between the 
greatest and the least magnitude of the star at different 
times. The interval between two successive times when 
the star is brightest is called the Period of Variation. 

73. Table of Variable Stars. — There are more than 100 
variable stars whose periods are known, besides others 
whose periods have not been determined. The following 
table contains a few of the former class : — 

Change of Magnitude, Period of 

from to Variation. 



46 years. 

n " 

435 days. 
331f " 
10 " 

99 « 



7] Argils, ... 1 4 . 

R Oephei, ...5 11. 

R Cassiopece, . . 6 lower than 14 . 
Oeti, . . . 1 or 2 lower than 1 1 . 

c Cancri, ... 8 10J- . 

(3 Tersei, . . . 2-j- . . . . . 4 . 

74. Mira. — The fourth star in the above table, called 
also Mira, or " the marvellous," has been known as a vari- 
able for about three centuries. It preserves its greatest 

of the smaller of the two stars in 61 Cygni f 71. What is meant by Variable 
Stars ? When the increase and decrease have been sudden, what have variables 
been called ? 72. By what is the variation in brightness measured ? What ia 
meant by the Period of Variation ? 73. How many variable stars are there, 
whose periods are known ? Mention one or two, with their change of magnitude 



40 THE STAES. 

brilliancy for about fifteen days, generally appearing at 
that time as a star of the first or second magnitude, but 
occasionally not brighter than one of the fourth. For the 
next three months its light decreases till it becomes invis- 
ible, not only to the naked eye, but even in telescopes of 
small power. It so remains for five months, then reap- 
pears, and in about three months again attains its maximum 
brightness, to repeat the same phases. Irregularities have 
been discovered in its period of variation, but these irreg- 
ularities are themselves periodical. 

75. Algol. — Among the variables, Beta ((3) Persei, or 
Algol, is perhaps the most interesting, as its period is 
short, and it never becomes invisible to the naked eye. It 
shines as a star of the second magnitude for two days 
thirteen hours and a half, and then suddenly loses its light, 
and in three hours and a half falls to the fourth magnitude ; 
its brilliancy then increases again, and in another period 
of three hours and a half it reattains its greatest brightness 
— all the changes being accomplished in less than three 
days. 

j 6. New, or Temporary, Stars. — Among the New, or 
Temporary, Stars, those observed in 1572 and 1866 are the 
most noticeable. The first appeared suddenly in the sky 
and was visible for seventeen months. Its light at first was 
equal to that of the planets at their greatest brilliancy ; so 
bright was it, indeed, that it was clearly visible at noon- 
day. Now, it is not a little curious that in the years 945 
and 1264 something similar was observed in the same re- 
gion of the sky (in Cassiopea) in which this star appeared. 
If, then, we assume that we have here a variable star of 
long period, which is very bright at its maximum and 
fades out of view at its minimum, we may expect a reap- 
pearance of the star about the year 1885. 

and period of variation. 74. Give an account of Mira. 75. Describe the changes 
of Algol. Point to Mira and Algol on the Celestial Charts. 76. Which are the 
most noticeable of the New, or Temporary, Stars ? Give an account of the one 



CAUSE OF VARIATION IN BRIGHTNESS. 41 

We now come to the new star which broke upon our 
sight in 1866, in the constellation Corona Borealis, and 
which was observed with powerful methods of research 
not employed before. This star was recorded some years 
ago as one of the ninth magnitude. In May, however, it 
suddenly flashed up, and on the 12th of that month shone 
as a star of the second magnitude. On the 14th it de- 
scended to the third magnitude ; the decrease of bright- 
ness was for some time at the rate of about half a magni- 
tude a day, but toward the end of May it was less rapid. 
There is good reason to believe that this increased bril- 
liancy was due to the sudden ignition of hydrogen gas in 
the star's atmosphere. Here we have a fact of the high- 
est importance, though as yet we can hardly do more than 
speculate upon it. 

j j. Cause of Variation. — The cause of this change of 
brightness in variable stars is one of the most puzzling 
questions in the whole domain of Astronomy. Three the- 
ories have been advanced : — 

1. That the variable revolves on its axis ; that its sur- 
face is not equally luminous in all parts ; and hence that 
it appears more or less bright, according to the part that 
is presented toward us. 

2. That the variable is accompanied by non-luminous 
planets, which in the course of their revolution get be- 
tween us and the variable, and thus eclipse the latter 
either in whole or in part. 

3. The most recent theory is that of Balfour Stewart, 
deduced from his researches on the Sun, which is doubt- 
less a variable star. He has found that the approach of a 
planet to our Sun increases its brightness, especially in 
that part which is nearest to the planet. Hence he sup- 
poses that the variable has a large planet revolving round 

that appeared in 1572. Describe the new star that appeared in 1866. To what 
ie the increased brilliancy of this star attributed? 77. What three theories have 
been advanced, to account for the change of brightness invariable stars? 78. 



42 THE STAKS. 

it at a small distance ; that part of the star which is 
nearest the planet will then be more luminous than that 
which is more remote, and, as the planet revolves, an 
appearance of variation, with a period equal to that of the 
planet's revolution, will be presented to the observer. 

If we suppose the planet to have a very elliptical orbit, 
then for a long time it will be at a great distance from its 
primary, while for a comparatively short time it will be 
very near. We should, therefore, expect a long period of 
darkness, and a comparatively short one of intense light — 
precisely what we have in temporary stars. 

Colored Stars. 

78. The light of most of the stars is white ; but there 
are a number of Colored Stars, which shine with red, 
orange, purple, blue, or green light, — some but faintly 
tinged, and others having a very deep and decided hue. 
Of large stars of different colors we may give the follow- 
ing table : — 

Red . . Aldeb'aran, Anta'res, Betelgeuse. 

Blue . . Capella, Bigel, Bella 'trix, Pro'cyon, Spica. 

Geeen . Sirius, Vega, Altair, Beneb. 

Yellow . Arctu'rus. 

White . Beg'ulus, Benebola, Bbmalhaut, Polaris. 

79. Colored Double Stars. — It is in the double and 
multiple stars that the richest colors are presented; and 
in these we also frequently find striking contrasts. Thus, 
in the double star Iota (l) Cancri, the larger of the two is 
orange, the smaller blue. The triple star Gamma (y) 
Andromeda is formed of an orange-red sun, accompanied 
by two others of an emerald green. In Eta {r\) Cassiopem 
we have a large white star, with a companion of a rich 
ruddy purple. 

What color is the light of most stars? What is the color of some? Mention 
some of the large colored stars. 79. In what stars are the richest colors pre- 



COLOKED STAES. 43 

What wondrous coloring must be met with in the 
planets lit up by these glorious suns, one sun setting, say 
in clearest green, another rising in purple, or yellow, 
or crimson; at times two suns at once mingling their 
variously-colored beams ! A remarkable group in the 
Southern Cross produced on Sir John Herschel " the effect 
of a superb piece of fancy jewelry." It is composed of 
over- 100 stars, only seven of which exceed the tenth 
magnitude ; two of this group are red, two green, three 
pale green, and one greenish blue. 

80. Changes of Color. — In many cases, the colors of the 
stars have changed. If we go back to ancient times, we 
read that Sirius was fiery-red; it gradually faded to a 
pure white, and is now a decided green. Capella was 
also described as red ; it afterward became yellow, and is 
now a pale blue. 

In some variable stars the changes of color are very 
striking. In the new star of 1572, Tycho Brahe observed 
changes from white to yellow, and then to red ; and we 
may add that generally, when the brightness decreases, 
the star becomes redder. 

81. The variations which the stars undergo in bright- 
ness and color, while we can not yet speak with certainty 
as to their causes, indicate that incessant movement and 
change are going on in the distant regions of space. 

Structure of the Stars. 

82. The Photosphere. — We will now pass on to what 
is known of the physical constitution of the stars. In the 
first place, the stars, of whatever their interiors may be 
composed, present to us on their exteriors a bright sur- 
face, which is called the Photosphere ; outside of this 



sented? Give examples. What remarkable group in the Southern Cross is 
mentioned? 80. State some remarkable changes of color. In what stars are 
changes of color very striking? 81. What do the changes in the brightness and 
color of the stars indicate? 82. What is the Photosphere of a star? What is 



44 THE STARS. 

photosphere, as outside the surface of our Earth, is an 
atmosphere composed of vapors. The materials of the 
photospheres are intensely hot; so hot, that the metals 
and other substances of which they consist are in a liquid 
or vaporous state. 

We can render this intelligible by taking water and 
iron as examples. When both are in a solid state, we 
have ice and hard iron. If we apply heat, we melt both 
ice and iron ; but we find that it requires much more heat 
to convert the latter into a liquid state than it does the 
former — the melting-point of ice being 32 ° of Fahrenheit's 
thermometer, while that of iron is about 2,000°. Having 
reduced both to liquids, we may by additional heat turn 
the water into steam, and the molten iron into iron-vapor ; 
but again the heat needed to vaporize the iron is vastly 
greater than that required to vaporize the water — how 
much greater is not known, as the heat necessary to pro- 
duce iron-vapor exceeds our powers of measurement. So, 
also, does the heat present in the photospheres of the 
stars. 

83. Materials of the Photospheres. — Do we know any 
thing of the substances which throw out this heat and 
light ? Yes, a little. For instance : — 

Sirius contains sodium, magnesium, iron, and hydrogen. 
Vega " sodium, magnesium, and iron. 

Pollux " sodium, magnesium, and iron. 

Beta Pegasi " sodium, magnesium, and perhaps barium. 

It is remarkable that the elements most widely diffused 
among the stars, including hydrogen, sodium, magnesium, 
and iron, are among those most closely connected with the 
living organisms of our globe. 

We shall be able, when we come to examine the 

outside of this photosphere ? What is the temperature of the materials of the 
photosphere? Show the effect of heat, hy taking water and iron as examples. 
83. State the materials which the photospheres of several of the brightest stars 
are found to contain. What is remarkable with respect to these elements ? 84. 



CAUSES OF THEIK COLOR. 45 

structure of the nearest star — the Sun — to obtain a more 
detailed knowledge of the structure of the stars generally. 

84. Causes of Color in the Stars. — The vapors produced 
in the photospheres of the stars ascend to form atmos- 
pheres, and these atmospheres absorb, in part, the light 
given out by the photospheres. A piece of colored glass 
will teach us what is meant by the absorption of light, and 
how it produces color. Thus, green glass is green because 
it absorbs all other light but the green; it is a sort of 
sieve, which stops every ray of light except the green 
ones. So with glasses, solids, vapors, or liquids, of other 
colors. 

Now, the colors of the stars may be influenced, not only 
by the degree of heat in their photospheres, but by the 
amount of absorption in their atmospheres. Our Sun at 
setting, for instance, sometimes seems blood-red, in con- 
sequence of the absorption of our atmosphere ; if the 
absorption were in his own atmosphere, he would be 
blood-red at noonday. 

Concerning the causes which produce the changes in 
color and brightness, we must confess that, after all, we 
are yet ignorant. 

Star Groups and Clusters. 

85. Having now dealt with the peculiarities of indi- 
vidual stars, — their distance, arrangement, color, varia- 
bility, and structure, — we next come to the various assem- 
blages of stars observed in various parts of the heavens. 

86. Remarkable Star-groups. — In the double and 
multiple systems (Art. 66) we saw the first beginnings 
of the tendency of the stars to group themselves together. 
In some parts of our system this tendency is exhibited in 
a very remarkable manner, the beautiful group of the 

How is the color of the stars explained ? What is said of the cause of changes 
of color? 86. What tendency is seen in the double and multiple systems of 



46 THE STABS. 

Pleiades (which may be found on the Celestial Chart, in 
the constellation Taurus) affording a familiar instance. 
The six or seven stars visible to the naked eye become 
60 or 70 when viewed in the telescope. The Hyades 
(near the Pleiades, in Taurus), and Prsesepe, or "the Bee- 
hive," in Cancer, may also be mentioned. 

In other cases, the groups consist of an innumerable 
number of suns apparently closely packed together. That 
in the constellation Perseus appears like a nebula to the 
naked eye, but viewed through a telescope it is separated 
into stars, and forms one of the most beautiful objects 
in the heavens. Many others, scarcely less stupendous, 
though much fainter by reason of their greater distance, 
are revealed by the telescope. 

87. Assemblages of stars are divided into, 

1. Irregular Groups, generally more or less visible 

to the naked eye. 

2. Star-clusters, invisible to the naked eye, but 

which, in the most powerful telescopes, are 
seen to consist of separate stars. These are 
subdivided into Ordinary Clusters and 
Globular Clusters. 

Clusters and nebulae are designated by their number in the catalogues 
which have been made of them by different astronomers. The most 
important of these catalogues are those of Messier, Sir William Herschel, 
and Sir John Herschel. About 5,400 nebulae have been observed. 

88. Of the Ordinary Star-clusters, the magnificent ones 
in the constellations Libra and Hercules (represented in 
Figs. 1 and 2, on page 47) may be mentioned as among 
those which are best seen in telescopes of moderate power. 
The Globular Clusters are well represented by those in 
Serpens and Aquarius (see Figs. 4 and 5, p. 47). 

8q. Other Universes. — Some of the clusters which lie 



stars? In what groups is this tendency further exhibited? 87. Into what two 
classes are assemblages of stars divided ? How are clusters and nebulae desig- 
nated? 88. Mention some of the Ordinary Star-clusters. Mention two Globular 



OEDINAEY AND GLOBULAE CLUSTEES. 



47 




STAR- CLUSTERS. 



1. In Libra. 2. In Hercules. 3. In Capricornus. 4. In Serpens. 
5. In Aquarius. 6. In Gemini, 



48 STAE-CLUSTEES. 

out of our universe, and which we must regard as other 
universes, are at such immeasurable distances, and are 
therefore so faint, that even the most powerful telescopes 
fail to reveal their real shape and boundaries. There is a 
gradual fading away at the edge, the last traces of which 
appear either as a luminous mist or cloud-like filament, 
which becomes finer till it ceases altogether to be seen. 
The Dumb-Bell Cluster, in Vulpecula, and the Crab Clus- 
ter, in Taurus, both of which have been resolved into stars, 
are instances of this. 

It is proper to say, however, that some astronomers believe all the 
visible star-clusters and nebulae to belong to our star-system or universe, 
which, if this be so, must include within itself miniatures of itself on a 
greatly reduced scale. 

90. In some of these star-clusters, the increase of bright- 
ness from the edge to the centre is so rapid as to make it 
appear that the stars are actually nearer together at the 
centre than they are at the edge ; in fact, that there is a 
real condensation toward the centre. 



CHAPTER II. 

NEBULH. 

91. Nebulae under the Telescope. — The term nebula 
was formerly applied to every thing in the sky which ap- 
peared cloud-like to the naked eye or in a telescope. Every 
time, however, a new telescope more powerful than any 
before used was brought to bear on them, numbers of 
what were till then called nebulae, and about which as 



Clusters. 89. Describe some of the very distant clusters. Mention two of these. 
90. What would seem to follow from the increase of brightness toward the centre 
of some of the clusters ? 

91. To what was the term nebula formerly applied ? What revelations were 



NEBULAE. 



49 



nebulae nothing was known, were found to be star-clus- 
ters, some of them of very remarkable forms, so distant 
that the smaller telescopes, powerful though they were, 
had failed to resolve them into distinct stars. Now, this is 
what has happened ever since the discovery of telescopes. 
Hence it was thought by some that all the so-called neb- 
ulae were, in reality, nothing but distant star-clusters. 

92. One of the most important discoveries of modern 
times, however, has furnished evidence of a fact long ago 
conjectured by some astronomers — namely, that some of 
the nebulae are something different from masses of stars, 
and that their cloud-like appearance is due to something 
else besides their distance and the insufficient power of 
our telescopes. This discovery is so recent that there has 
not yet been time to sort out the real from the apparent 
nebulae. We are obliged, therefore, still to accept as neb- 
ulae all formerly classed as such which up to this time 
have not been resolved into stars. 

93. Classifi- 
cation. — Nebu- 
lae may be di- 
vided into five 
classes: — 1. Ir- 
regular Nebu- 
lae. 2. Ring 
and Elliptical 
Nebulae. 3. Spi- 
ral or Whirl- 
pool Nebulae. 4. 
Planetary Neb- 
ulae. 5. Nebulae 
surrounding 
stars. 




Fig. 21.— Great Nebtjxa of Orion. 



made, as more powerful telescopes were used? What inference was drawn 
from this ? 92. What has since been discovered respecting some of the nebulse ? 
93. Into how many classes may nebulas be divided ? Name them. 94. To which 
3 



50 



NEBULAE. 



94. Irregular Nebulae. — Some of the irregular nebulae 
are visible to the naked eye on a dark night. Among 
these is the great nebula of Orion (Fig. 21), in the part of 
the constellation occupied by the sword-handle and sur- 
rounding the multiple star Theta (6). The nebulosity 
near the stars has the appearance of separate flakes, and 
is of a greenish-white tinge. There seems no doubt 
that the shape of this nebula and the position of its bright- 
est portions are changing. One part of it appears, in a 
powerful telescope, startlingly like the head of a fish. On 
this account it has been termed the Fish-mouth Nebula. 

Two fine irregular nebulae are visible in the southern 
hemisphere : one is in the constellation Dorado, the other 
surrounds Eta (rf) Argils. The latter occupies a space 
equal to about five times the apparent area of the Moon. 

95. Ring and Elliptical Nebulae. — We have classed the 
ring and elliptical nebulae together, because probably the 
latter are ring-nebulae looked at sideways. The finest 

ring-nebula is 
in the con- 
stellation Ly- 
ra, not far 



1 

; 


% 




w 



Fig. 22.— Rikg-Nebula in Lyba. 



from the star 
Vega. As 
seen by Sir 
John Her- 
schel, it pre- 
sented the ap- 
pearance of an oval ring surrounding a darker space (see 
Fig. 22, No. 1), the uniform pale glimmer of which resem- 
bled a light gauze stretched across the ring. Lord Rosse's 
more powerful telescope has since partially resolved the 

of these classes does the great nebula of Orion belong ? Describe this nebula. 
What name has been given to it, and why ? What irregular nebulae are visible 
in the southern hemisphere? 95. Why are the Eing and Elliptical Nebulae 
classed together? Which is the finest ring-nebula? Describe it, as seen by Sir 
John Herschel. As seen through Lord Rosse's telescope. Where is there a fine 



SPIRAL NEBULJE. 



51 




ring into luminous points (see Fig. 
22, No. 2), and has shown parallel 
lines in the opening and a fringe 
of light about the outside border. 

Near the beautiful triple star 
Gamma (y) Andromedm is a fine 
specimen of an elliptical nebula, 
having two stars near the extremi- 
ties of the major axis of the el- _ .. _ 

J Fig. 23.— Elliptical Nebula 

lipse. near y Andromeda,. 

96. Spiral Neb- 
ulae. — The spiral or 
whirlpool nebulae 
are represented by 
that in the constel- 
lation Canes Vena- 
tici. In an ordi- 
nary telescope it 
presents the ap- 
pearance of two 
globular clusters, 
one of them sur- 
rounded by a ring 
at a considerable 
distance, the ring- 
varying in bright- 
ness, and being 
divided into two 
in a part of its length. But in a larger instrument the 
appearance is entirely changed. The ring turns into a 
spiral coil of nebulous matter, and the outlying mass is 
seen connected with the main mass by a curved band. 

In the constellations Pisces and Yirgo we have other 
examples of this strange phenomenon (the 33d and 99th 

specimen of an elliptical nebula ? 96. In what constellation is there a remarkable 
spiral nebula ? Describe it. In what other constellations do spiral nebulae 




Fig. 24.— Spiral Nebula in Canes Venatici. 



52 



NEBULAE. 




Fig. 25.— Planetary Neb 
ula in Ursa Major. 



in Messier' s catalogue), which indicate the action of stu- 
pendous forces of a kind unknown in our own universe. 

97. Planetary Nebulae. — These 
were so called by Sir John Herschel. 
They are circular or slightly elliptical 
in form, and shine with a planetary 
and often bluish light. One in Ursa 
Major will serve as a specimen. 

98. Nebulse surrounding Stars. — 
We come lastly to the nebulae sur- 
rounding stars, or nebulous stars. 
The stars thus surrounded are ap- 
parently like all other stars, save in the 

fact of the presence of the appendage ; nor 

does the nebula give any signs of being 

resolvable with our present telescopes. 

Iota (1) Orionis, JEpsilon (e) Orionis, 8 

Canum Venaticorum, and 79 Ursce Ma- 

joris, belong to this class. 

99. Brightness of the Nebulse. — Like 
the stars, the nebulae differ in brightness, but as yet they 
have not been divided into magnitudes. This, however, 
has been done in a manner by determining the space- 
penetrating or light-grasping power of the telescopes 
powerful enough to render them visible. 

Thus, it has been estimated that Lord Rosse's great 
Reflector, the most powerful instrument as yet used in 
such inquiries, penetrates 500 times farther into space 
than the naked eye can ; hence a nebula which this tele- 
scope just renders visible must be 500 times farther off 
than a star of the sixth magnitude. Now, as light re- 
quires 120 years to reach us from such a star, the tele- 
occur? 97. From whom did the planetary nebulae receive their name, and why ? 
What is their form? Where does one occur? 98. What is the last class of 
nebulae ? Describe the nebulous stars. Mention four of this class. 99. How 
do the nebulae compare with each other in brightness ? How has their magnitude 
in a manner been determined? Illustrate this in the case of Lord Eosse's tele- 




Fig. 26.— Nebulous 
Star, i Ononis. 



VAEIABLE NEBULJS. 53 

scope referred to penetrates so profoundly into space that 
no star can escape its scrutiny, unless at a distance that it 
would take light sixty thousand years to traverse. 

An idea of the extreme faintness of the more distant 
nebulae may be gathered from the fact, that the light of 
some of those visible in an instrument of moderate size 
has been estimated to range from lg 1 00 to -g-owo °f tne 
light of a sperm-candle consuming 158 grains of material 
per hour, viewed at the distance of a quarter of a mile ; 
that is, such a candle a quarter of a mile off is from 
1,500 to 20,000 times more brilliant than these nebulw. 

100. Variable Nebulae. — The phenomena of variable, 
lost, new, and temporary stars, have their equivalents in 
the case of the nebulae, the light of which, it has been 
lately discovered, is in some cases subject to great varia- 
tions. 

In 1861 it was found that a small nebula, discovered 
in 1856 in Taurus, near a star of the tenth magnitude, had 
disappeared, the star also becoming dimmer. In the next 
year the nebula regained its brightness. Another nebula, 
which in May, 1860, appeared as a star of the seventh 
magnitude, during the next month recovered its nebulous 
appearance. 

101. Distribution of the Nebulae. — In Art. 48 the 
marked character of the distribution of the stars of our 
universe, giving rise to the appearance of the Milky Way, 
was pointed out. The distribution of the nebulae, how- 
ever, is very different ; in general, they lie out of the 
Milky Way, so that they are either less condensed there, 
or the visible universe (as distinguished from our own 
stellar universe) is less extended in that direction. They 
are most numerous in a zone which crosses the Milky Way 
at right angles, the constellation Virgo being so rich in 

scope. How does the light of some nebulas visible through a telescope of 
moderate size compare with that of a canrile ? 100. Give examples of variable 
nebulas. 101. How are the nebulas distributed ? Where are they most numerous ? 



54 NEBULA. 

them that a portion of it is termed the nebulous region of 
Virgo. In fact, not only is the Milky Way the poorest in 
nebulae, but the parts of the heavens farthest from it are 
the richest. 

102. Physical Constitution of the Nebulae. — We now 
come to the question, What is a nebula ? The answer is — 
A true nebula is a mass of incandescent or glowing gas, 
and there are indications that the gases in question are 
nitrogen and hydrogen. This fact, the fruit of the brilliant 
discovery before alluded to (Art. 92), forever sets at rest 
the question so long debated, as to the existence of a 
Nebulous Fluid in space. 

When, therefore, we see closely-associated points of 
light in a nebula, we must not suppose that the latter is 
necessarily resolvable into stars. These luminous points, 
in some nebulae at least, must be looked upon as them- 
selves gaseous bodies, denser portions probably of the 
great nebulous mass. It has been suggested that the 
apparent permanence of general form in a nebula is kept 
up by the continual motions of these denser portions. 

103. The Nebular Hypothesis, given to the world before 
the existence of a nebulous fluid was proved, supposes that 
there once existed in space a great, chaotic, nebulous 
mass, endowed with a kind of whirlpool motion, which, 
gradually condensing through the mutual attraction of its 
particles, formed the countless suns distributed through 
space ; that the planets were formed by the condensation 
of rings of matter successively thrown off by the central 
mass, and the satellites by the condensation of matter 
thrown off in like manner by their primaries. It may 
take years to prove, or disprove, this hypothesis ; but the 
tendency of recent observations is to show its correctness. 



102. Of what is a nebula composed ? When we see closely-associated points of 
light in a nebula, what must we not suppose? What may these luminous points 
be ? 103. What is the substance of the Nebular Hypothesis ? What is the bear- 
ing of recent observations ? 



THE SUN— THE NEAREST STAB. 55 

CHAPTER III. 
THE SUN. 

104. The Sun. — We shall now consider the star nearest 
to us, which dazzles the whole family of planets by its 
brightness, supports their inhabitants by its heat, and 
keeps them in bounds by its weight. In almanacs and 
astronomical treatises, the Sun is denoted by either of the 
following signs : or§. 

105. The Sun's Disk. — The Disk of a heavenly body is 
its face, as it appears projected on the sky. The Sun's 
disk is a perfect luminous circle. Hence, as we know 
that the Sun revolves on its axis (Art. 110), we conclude 
that its form is that of a perfect sphere. 

The Sun's disk varies slightly in size, according to the 
Earth's distance from the Sun, being largest about January 
1st, when we are nearest to it, and smallest about July 1st, 
when we are farthest off. If the mean size of the disk 
(presented to us about the 1st of April and October) be 
represented by 100, its greatest size will be 107, and its 
least 94. 

106. Relative Brilliancy and Size. — The brilliancy of 
the Sun, compared with that of the other stars, is so great 
that it is difficult at first to look upon it as in any way 
related to those feeble twinklers. This difficulty, however, 
is soon dispelled when we consider that its distance from 
us is less than -g-owo 0" °f tnat °f tne nearest star, Alpha 
(a) Gentauri. Removed as far as the latter is from us, 
our Sun would be a star of the second magnitude ; and, 
removed to the mean distance of the first-magnitude stars, 

104. What are we next to consider? By what signs is the Sun denoted? 
105. What is meant hy the Disk of a heavenly body ? What is the Sun's disk? 
Hence, as we know that the Sun turns on its axis, what do we conclude respect- 
ing its shape? When is the Sun's disk largest, when smallest, and why? 106. 
How does the light of the Sun compare with that of the stars ? How is this 
difference explained ? How would the Sun look, if removed to the mean dis- 



56 THE SUN. 

it would be just visible to the unaided sight as a star of the 
sixth magnitude. 

Our Sun is, therefore, by no means one of the largest 
stars. If we assume that the light given out by Sirius, for 
instance, is no more brilliant than our sunshine, that star 
would be equal in bulk to more than 3,000 Suns. 

107. Distance and Diameter. — The mean distance of the 
Sun from the Earth is now known to be about 91,000,000 
miles. These figures, as in the case of the distances of 
the stars, fail to convey any definite idea to "the mind. 
Were there a railroad from the Earth to the Sun, a train 
going night and day at the rate of 30 miles an hour, and 
starting on the 1st of January, 1870, would not reach the 
Sun till about the middle of the year 2208. 

108. The Sun's distance being known, it is easy to 
determine its size. The distance from one side of the Sun 
to the other, through its centre — or, in other words, the 
diameter of the Sun, — is 852,584 miles. If the Sun were 
so placed that its centre coincided with that of the Earth, 
this immense luminary would not only fill the whole orbit 
of the Moon, but extend beyond it three-fourths of the 
Moon's distance from the Earth. A train going at the 
speed named above would accomplish the journey round 
our Earth in a little over a month ; a railway journey 
round the Sun, the same speed being maintained, would 
require more than ten years. 

If we represent the Sun by a globe about two feet in 
diameter, a pea at the distance of 430 feet will represent 
the Earth ; and the nearest fixed star would be represented 
by a similar globe placed at the distance of 9,000 miles. 

109. Volume and Mass. — More than 1,200,000 Earths 
would be required to make one Sun. Astronomers ex- 

tance of the 1st -magnitude stars? How does the Sun compare in size with 
Sirius? 107. How far is the Sun from the Earth? Give some idea of this dis- 
tance, by telling how long it would take to travel it by rail. 108. What is the 
length of the Sun's diameter? Give an idea of this distance. How may we 
represent the Sun, the Earth, and the nearest fixed star? 109. What is the dif 



EOTATION OF THE SUN. 57 

press this by saying that the volume of the Sun is over 
1,200,000 times greater than that of the Earth. But as 
the matter of which the Sun is composed weighs only one- 
quarter as much, bulk for bulk, as that of the Earth, 
300,000 Earths only would be required in one scale of 
a balance to weigh down the Sun in the other. That is, 
the mass, or weight of the Sun, is 300,000 times greater 
than that of our Earth. 

1 10. Rotation. — The Sun, like the Earth or a top when 
spinning, turns round on an axis ; this rotation was dis- 
covered by observing the spots on its surface, about which 
we shall presently have much to say. It is found that the 
spots always make their first appearance on the same side 
of the Sun ; that they travel across it in from twelve and 
a half to fourteen days, and then disappear on the other 
side. This is not all : if they be observed in June, they 
go straight across the sun's disk with a dip downward ; if 
in September, they cross in a curve ; while in December 
they go straight across again, with a dip upward, and in 
March their paths are again curved, but this time in the 
opposite direction. 

June. September. December. March. 




Fig. 21. — Apparent Paths of the Spots across the Sun's Disk, as seen 
from the Earth at different times of the year. The arrows show the 
direction in which the Sun rotates. 

111. The Plane of the Ecliptic. — It is important that 

ference between volume and mass ? How does the Sun compare with the Earth 
in volume ? How, in mass ? 110. What has been found, by observing the spots 
on the Sun ? What appearances do these spots present ? 111. What is meant by 



58 THE SUN. 

we make this perfectly clear. We know that the Earth 
goes round the Sun once a year. It has been found, also, 
that its path is level — that is to say, the Earth in its 
journey does not go up or down, but always straight on; 
we may imagine it as floating round the Sun on a bound- 
less ocean, in which both Sun and Earth are half immersed. 
We shall see further on that this level — called the Plane 
of the Ecliptic — is used by astronomers in precisely the 
same way as we commonly use the sea-level. We say, for 
instance, that such a mountain is so high above the level 
of the sea. Astronomers say that such a star is so high 
above the plane of the ecliptic. 

1 1 2. Inclination of the Sun's Axis. — We have imagined 
the Earth and Sun to be floating in an ocean up to the 
middle. Now, if the Sun were quite upright, the spots 
would always seem at the same distance above the level 
of our ocean. But this, we have found, is not the case. 
From the two opposite points of the Earth's path which 
it occupies in June and December, the spots are seen to 
describe straight lines across the disk, while midway be- 
tween these points (in September and March) their paths 
are observed to be decided curves, rounding downward in 
the one case and upward in the other (see Fig. 27). A mo.- 
ment's thought will show that these appearances can arise 
only from an inclination in the Sun's axis. The Earth in 
its annual revolution attains in September a point at which 
the Sun's axis is inclined toward it ; and in March reaches 
the opposite point of its orbit, at which the Sun's axis is 
inclined away from it. 

113. Time of Rotation. — It has been found that the 
spots, besides having an apparent motion, caused by their 
being carried round by the Sun in its rotation, have a mo- 



the Plane of the Ecliptic ? What use is made of it by astronomers ? 112. What 
is found to be the case, with regard to the Sun's axis ? How is this inclination 
proved ? Why do the paths of the spots carve in different directions in September 
and March? 113. What motion have the spots besides their apparent motion? 



PEEIOD OF ITS ROTATION. 59 

tion of their own. This proper motion, as distinguished 
from their apparent motion, has recently been thoroughly 
investigated, and accounts for the great difference in the 
periods which different observers have assigned to the 
Sun's rotation. As already stated, this rotation has been 
deduced from the time taken by the spots to cross the 
disk ; but it now seems that all sun-spots have a motion 
of their own, the rapidity of which varies regularly with 
their distance from the solar equator — that is, the line 
half-way between the two poles of rotation. The spots 
near the equator travel faster than those away from it : so 
that, if we take an equatorial spot, we shall say that the 
Sun rotates in about twenty-five days ; whereas, if we take 
one half-way between the equator and the poles, in either 
hemisphere, we shall say that it rotates in about twenty- 
eight days. 

We are still, therefore, ignorant of the exact time of 
the Sun's rotation ; for, if it is a solid mass, it can of course 
have but one period — and which of the two named above 
it may be, if either of them, we have no means of telling. 

114. Telescopic Appearance. — We have now considered 
the distance and size of the Sun ; we have found that, like 
our Earth, it rotates on its axis, and we have determined 
the direction in which the axis points. We must next try 
to learn something of the appearance it presents when 
viewed through a telescope, and of its nature or physical 
constitution. On this latter point our knowledge is not 
yet complete. This, however, is little to be wondered at. 
We have gleaned so many facts, at stupendous distances 
the very statement of which is almost meaningless to us, 
that we forget that our mighty Sun, in spite of its brilliant 
light and fostering heat, is still some 91,000,000 miles 

For what does this proper motion account? How do the sun-spots differ, as 
regards their proper motion? What is the exact time of the Sun's rotation? 
114. What keeps us from knowing more about the physical constitution of the 
Sun ? What caution is given, with respect to looking at the Sun ? 115. What 
are the first things that strike us, on looking at the Sun through a powerful 



60 



THE SUN. 



away ; and that, even though we employ the finest tele- 
scope, we can only observe the various phenomena as we 
should do with the naked eye at a distance of 180.000 
miles. 

To look at the Sun through a telescope, without proper appliances, is 
a very dangerous affair. Several astronomers have lost their eyesight by 
so doing, and the student should not use even the smallest telescope with- 
out proper guidance. 

115. Sun-spots. — The first things which strike us on 
the Sun's surface, when we look at it with a powerful tele- 
scope, are dark spots. On the opposite page we give 
drawings of a very fine one, visible on the Sun in 1865. 
The spots are not scattered all over the Sun's disk, but 
are generally limited to those parts of it a little above and 
below the Sun's equator, which is represented by the mid- 
die lines in Fig. 27. 

1 1 6. The spots float, as it were, in what, as we have 
already seen in the case of the stars, is called the photo- 
sphere; the half-shade shown in the spot is called the 
penumbra; inside the penumbra is a still darker shade, 
called the umbra, and inside this again is the nucleus. 
Figs. 3 and 4 on the opposite page will render this per- 
fectly clear. The white surface is the photosphere; the 
half-tones represent the penumbra; the dark, irregular 
central portions, the umbra ; and the blackest parts in the 
centre of these dark portions, the nucleus. 

1 1 7. Sun-spots are cavities, or hollows, in the photo- 
sphere, and these different shades represent different 
depths. 

118. Diligent observation of the umbra and penumbra, 
with powerful instruments, reveals to us the fact that 
change is incessantly going on in the region of the spots. 
Sometimes changes are noticed even within an hour : here 

telescope ? How are these spots situated ? 116. What is the Photosphere ? 
The Penumbra ? The Umbra ? The Nucleus ? 117. What are Sun-spots ? 118, 
What is constantly going on in the region of the spots ? Describe some of these 



SUN-SPOTS. 



61 







"* 





$, 






.-Pit 






The Great Sun-spot op 1865. 
1. The spot entering the Sun's disk, Oct. 7th (foreshortened view). 2. 
Its appearance, Oct. 10th. 3. Central view, Oct. 14th, showing the for- 
mation of a bridge, and the nucleus. 4. Its appearance, Oct. 16th. 



62 



THE SUN. 



part of the penumbra is seen sailing across the umbra; 
here a portion of the umbra is melting from sight ; here, 
again, is an evident change of position and direction in 
masses which retain their form. The enormous changes, 
extending over tens of thousands of square miles of the 
Sun's surface, which took place in the great sun-spot of 
1865, are represented in the diagrams on page 61. 

l i 9. Faculse. — Near the edge of the solar disk, and es- 
pecially about spots approaching the edge, it is quite easy, 
even with a small telescope, to discern certain very bright 
streaks of diversified form, quite distinct in outline, and 
either entirely separate or uniting in various ways into 
ridges and net-work. These appearances, which have been 



> 



C;'W''W§?" :: ' 






t §81 Hit 







Fig. 28.— Sun-spots and Faculse. P/om a Photograph. 

termed JFhculce, are the most brilliant parts of the Sun. 
Where, near the edge, the spots become invisible, undu- 
lated shining ridges still indicate their place — being more 
remarkable there than elsewhere, though everywhere 
traceable in good observing weather. Faculae may be 

changes. 119. What are Faculse ? What is said of their size ? How do they 
sometimes lie, as regards spots ? 120. How does the Sun's surface look, where 



APPEAEANOES ON THE SUN'S DISK. 



63 



of all magnitudes, from hardly-visible, softly-gleaming, 
narrow tracts 1,000 miles long, to continuous complicated 
ridges 40,000 miles and more in length, and from 1,000 to 
4,000 miles broad. Ridges of this kind often surround a 
spot, and hence appear the more conspicuous; such a 
ridge is shown in Fig. 1, page 61. Sometimes there ap- 
pears a very broad white platform round the spot, and 
from this white crumpled ridges pass in various directions. 
i 20. Other Appearances on the Sun's Disk. — The whole 
surface of the Sun, except those portions occupied by the 
spots, is coarsely 



mottled ; and, in- 
deed, the mottled 
appearance requires 
no very great opti- 
cal power to render 
it visible. Viewed 
through a large tel- 
escope, the surface 
seems to be made 
up principally of lu- 
minous masses, 
called by Sir Wil- 
liam Herschel cor- 
rugations, and de- 
scribed by other ob- 
servers as resem- 
bling " rice-grains," 
"granules," etc. 
i2i. The term 




Fig. 29. — " Willow-leaves " in a Sun-spot. A, 
tongue of facula stretching out into the umbra. 
B, clouds. C, layers of " willow-leaves " in the 
penumbra. 



willow-leaves has been appropriately applied to appear- 
ances sometimes observed in the penumbrae of spots. 
They consist of elongated masses of unequal brightness, 



ii; is not covered with spots ? Of what does it seem to be made up, when viewed 
through a large telescope? 121. What is meant by willow-leaves f 122. What is 



64 THE SUN. 

so arranged that for the most part they point like so many 
arrows to the centre of the nucleus, giving to the penum- 
bra a radiated appearance. At other times, and occasion- 
ally in the same spot, the jagged edge of the penumbra 
projecting over the nucleus has caused the interior edge 
of the penumbra to be likened to coarse thatching with 
straw. 

122. There are darker or shaded portions between the 
granules, often pretty thickly covered with dark dots, like 
stippling with a soft lead-pencil ; these are what have 
been called pores by Sir John Herschel, and punctulations 
by his father. Some of these are almost black, and are 
like excessively small eruptive spots. 

123. When the Sun is totally eclipsed, — that is, as will 
be explained by-and-by, when the Moon comes exactly 
between the Earth and the Sun, — other appearances are 
unfolded to us, which the extreme brightness of the Sun 
prevents our observing under ordinary circumstances. 
The Sun's atmosphere is then seen to contain red masses 
of fantastic shapes, some of them quite disconnected from 
the Sun ; to these the names of red-flames and prominences 
have been given. Now, as these bodies appear much 
brighter than the surrounding atmosphere, we conclude 
that they are hotter than the latter, as a bright fire is 
hotter than a dim one. 

124. Explanation of the Appearances on the Sun's 
Disk. — Let us see if we can account for the appearances 
which the Sun's disk presents, when viewed through a 
powerful telescope. As the spots break out and close up 
with great rapidity, as changes both on a large and a 
small scale are constantly taking place on the surface, we 
can only infer that the photosphere of the Sun, and there- 



meant by pores or punctulations ? 123. What appearances are presented when 
the Sun is totally eclipsed ? What do we conclude, with respect to these appear- 
ances? 124. To explain the appearances on the Sun's surface, what is supposed 
respecting the photosphere ? What further seems to be the case, as regards the 



SUN-SPOTS, FACUL.E, ETC., EXPLAINED. 65 

fore of the stars, is of a cloudy nature. But while our 
clouds are made up of particles of water, the clouds on the 
Sun must be composed of particles of various metals and 
other substances in a state of intense heat. The photo- 
sphere is surrounded by an atmosphere composed of the 
vapors of the bodies which are incandescent in the 
photosphere. 

It seems, also, that not only is the visible surface of 
the Sun entirely of a cloudy nature, but that the atmos- 
phere is a highly-absorptive one. Thus when the clouds 
are highest they appear brightest — we seefaculw — because 
they extend high into the atmosphere, and consequently 
there is less atmosphere to obscure our view. Spots may 
be due to the absorption of a greater thickness of atmos- 
phere, as they are hollows in the cloudy surface ; or the 
whole of the cloudy surface may be cleared off in those 
parts from a surface beneath, which emits less light than 
the clouds. 

The more minute features — the granules — are most 
probably the dome-like tops of the smaller masses of 
cloud, bright for the same reason that the faculse are 
bright, but in a less degree. The fact that these granules 
lengthen out as they approach a spot and descend the 
slope of the penumbra, may be accounted for by supposing 
them to be elongated by the current which draws them 
down into a spot, as the clouds in our own sky are length- 
ened out when they are drawn into a current. 

125. The Sun, a Variable Star. — Some spots cover 
millions of square miles, and remain for months ; others 
are visible only in powerful instruments, and are of very 
short duration. There is a great difference in the number 
of spots visible from time to time ; indeed, there is a 
minimum period, when none are seen for weeks together, 

solar atmosphere ? Under what circumstances do we see faculse ? To what are 
spots due? What are the granules? How is their lengthening out as they 
approach a spot accounted for? 125, What is found to he the case, as regards 



66 THE SUN. 

and a maximum period, when more are seen than at any- 
other time. The interval between two maximum or two 
minimum periods is about eleven years. 

Now, as we must get less light from the Sun when it is 
covered with spots than when it is free from them, we 
may look upon it as a variable star, with a period of 
eleven years. 

It has recently been shown that this period is in some 
way connected with the action of the planets on the 
photosphere. It is also known that the magnetic needle 
has a period of the same length, its greatest oscillations 
occurring when there are most sun-spots. Auroras, and 
the currents of electricity which traverse the Earth's 
surface, are affected by a similar period. There seems, 
therefore, to be some connection between these things and 
the solar spots, though what it is we do not know. 

126. Elements in the Sun. — We have before seen (Art. 
83) what substances exist in a state of incandescence in 
some of the stars. In the case of the Sun we are ac- 
quainted with a greater number. Here is the list : — 

Sodium. Zinc. Gold, probable. 

Iron. Calcium. Cobalt, doubtful. 

Magnesium. Chromium. Strontium, ditto. 

Barium. Nickel. Cadmium, ditto. 

Copper. Hydrogen, probable. Potassium, ditto. 

The atmosphere of the Sun, like that of the stars, consists 
of the vapors of these and of other — yet unknown — sub- 
stances, and extends to a height exceeding 80,000 miles 
above the visible surface. 

127. Benign Influences of the Sun. — Let us now inquire 
into some of the benign influences spread broadcast by 
the Sun. We all know that our Earth is lit up by its 

the size and duration of the sun-spots ? As regards the periods of their occur- 
rence ? What conclusion is drawn respecting the Sun ? With what does the 
occurrence of solar spots seem to he connected? 126. Mention some of the 
elements known to exist in the Sun. Of what does the solar atmosphere consist ? 



SOLAR LIGHT AND HEAT. 67 

beams, and that we are warmed by its heat; but this by 
no means exhausts its benefits, which we share in common 
with the other planets that gather round its hearth. 

1 28. And first, as to its light. We have already com- 
pared its light with that which we receive from the stars, 
but that is merely its relative brightness ; we want now to 
know its actual or intrinsic brightness. It is clear, at once, 
that no number of candles can rival this brightness ; let 
us therefore compare it with one of the brightest lights 
that we know of — the calcium light. The calcium light 
proceeds from a ball of lime made intensely hot by a flame 
composed of a mixture of hydrogen and oxygen playing 
on it. It is so bright, that we cannot look on it any more 
than we can on the Sun ; but if we place it in front of the 
Sun, and look at both through a dark glass, the calcium 
light, though so intensely bright, looks like a black spot. 
In fact, Sir John Herschel has found that the Sun gives 
out as much light as 146 calcium lights would do, if each 
ball of lime were as large as the Sun and gave out light 
from all parts of its surface. 

129. Then, as to the Sun's heat. The heat thrown out 
from every square yard of the Sun's surface is greater 
than that which would be produced by burning six tons 
of coal on it each hour. Now, we may take the surface 
of the Sun roughly at 2,284,000,000,000 square miles, and 
there are 3,097,600 square yards in each square mile. 
How many tons of coal must be burnt, therefore, in an 
hour, to represent the Sun's heat ? 

130. But the Sun sends out, or radiates, its light and 
heat in all directions; it is clear, therefore, that as our 
Earth is so small compared with the Sun, and is so far 
away from it, the light and heat the Earth can intercept 
is but a very small portion of the whole amount ; in fact, 

128. How does the brightness of the Sun compare with that of a calcium light? 

129. Give some idea of the Sun's heat. 130. How much of this does the Earth 
get? How much do all the planets together receive? What would be the effect 



68 THE SUN. 

we only grasp the ^to o",¥o"o,oto" P art °f ^- All' the planets 
together receive but one 2 2 7 -millionth part of the solar 
light and heat. 

The whole heat of the Sun collected on a mass of ice as 
large as the Earth would be sufficient to melt it in two 
minutes, to boil the water thus produced in two minutes 
more, and to turn it all into steam in a quarter of an hour 
from the time it was first applied. 

131. But this is not all. There is something else be- 
sides light and heat in the Sun's rays, and to this some- 
thing we owe the fact that the Earth is clad with verdure ; 
that in the tropics, where the Sun shines always in its 
might, vegetable life is most luxuriant, and that with us 
the spring-time, when the Sun regains its power, is marked 
by a new birth of flowers. There comes from the Sun, 
besides its light and heat, chemical force, which separates 
carbon from oxygen, and turns the gas which, were it 
to accumulate, would kill all men and animals, into the 
life of plants. Thus, then, does the Sun build up the 
vegetable world. 

132. Let us go a step farther. The enormous engines 
which do the heavy work of the world, — the locomotives 
which take us so smoothly and rapidly across a whole 
continent, — the mail-packets which bear us so safely over 
the broad ocean, — owe all their power to steam, and 
steam is produced by heating water by coal. We all 
know that coal is the product of an ancient vegetation ; 
and vegetation is the direct effect of the Sun's action. 
Hence, without the Sun's action in former times we should 
have had no coal. The heavy work of the world, there- 
fore, is indirectly done by the Sun. 

133. Now for the light work. Let us take man. To 
work, a man must eat. Does he eat beef? On what was 

of (he whole heat of the Sun, collected on a mass of ice as large as the Earth ? 
131. What else, besides light and heat, do we owe to the Sun ? What is the 
effect of this chemical force? 132. Show how the heavy work of the world is 



FUTURE OF THE SUN. 69 

the animal which supplied the beef fed ? On grass. Does 
he eat bread ? Of what is bread made ? Of the flour of 
wheat and other grains. In these, and in all cases, we 
come back to vegetation, which is, as we have already 
seen, the direct effect of the Sun's action. Here again, 
then, we must confess that to the Sun is due man's power 
of work. In fact, all the world's work, with one trifling 
exception (tide-work, of which more hereafter), is done by 
the Sun ; and man himself, prince or peasant, is but a little 
engine, which merely directs the energy supplied by the 
Sun. 

134. Is the Sun inhabited? — This is a question more 
easily asked than answered. If the whole body of the 
Sun is an incandescent globe, of course no organized 
beings of whom we can conceive can live upon it. But if 
the incandescence is confined to its photosphere, as many 
think, and the surface of the globe itself is protected from 
its outer envelope by a dense atmosj^here, which absorbs 
its intense light and is at the same time a non-conductor 
of heat, there is nothing to prevent it from being inhabited. 

135. The Future of the Sun. — Will the Sun keep up 
forever a supply of the force that has been described ? 
It cannot, if it be not replenished, any more than a fire 
can be kept in unless we put on fuel ; any more than a 
man can work without food. At present, philosophers 
know not by what means it is replenished. As, probably, 
there was a time when the Sun existed as matter diffused 
through infinite space, the condensation of which matter 
has stored up its heat, so, probably, there will come a time 
when the Sun, with all its planets welded into its mass, 
will roll, a cold, black ball, through infinite space. 

We have no evidence, however, of any loss of heat, 
even from century to century; and, if there is a loss, 



done by the San. 133. Show how man's power of work is due to the Sun. 134. 
Is the Sun inhabited ? 135. What is the probable future of the Sun ? 136. 



70 THE SOLAE SYSTEM. 

there will doubtless be sufficient beat left to supply the 
planets with all they need for thousands of years to come. 

136. Such, then, is our Sun — the nearest star. Although 
some of the stars do not contain those elements which on 
the Earth are most abundant (a Orionis and (3 Pegasi, for 
instance, are worlds without hydrogen), still we see that, 
on the whole, the stars differ from each other, and from 
our Sun, only in special modifications, and not in general 
structure. There is, therefore, a probability that they 
fulfil an analogous purpose; and are, like our Sun, sur- 
rounded with planets, which they uphold by their attrac- 
tion, and illuminate and energize by their radiation. 
Hence the probable past and future of the Sun are the 
probable past and future of every star in the firmament 
of heaven. 



CHAPTER IV. 

THE SOLAR SYSTEM. 

137. General Description. — From the Sun we now pass 
to the system of bodies which revolve round it ; and here, 
as elsewhere in the heavens, we come upon the greatest 
variety. We find planets — of which the Earth is one — 
differing greatly in size, and situated at various distances 
from the Sun. We find again a ring of little planets 
clustering in one part of the system; these are called 
asteroids, or minor planets : and we already know of at 
least two masses or rings of smaller planets still, some of 
them so small that they weigh but a few grains. These 
give rise to the appearances called meteors, bol'i-des, or 

Keasoning by analogy from the Sun, what may we suppose with respect to the 
stars ? 

137. What different bodies do we find in the Solar System ? 138. How many 



OF WHAT COMPOSED. fj 

shooting-stars. We find also comets, some of which break 
in upon us from all parts of space, and then, passing round 
our Sun, rush back again ; while others are so little erratic 
that they may be looked upon as members of the solar 
household. Besides these, there is another ring visible to 
us, under the name of the zodiacal light. 

138. In the Solar System, then, we have Eight large 
Planets, named as follows, in the order of their distance 
from the Sun, and denoted in Almanacs, etc., by the signs 
appended to them respectively : — 

. 1. Mercury, $ 5. Jupiter, % 

2. Venus, $ 6. Saturn, ^ 

3. Earth, © 7. Uranus, 1$ 

4. Mars, $ 8. Neptune, f 

One hundred and forty small Planets revolving round 
the Sun between the orbits of Mars and Jupiter. They 
are denoted by numbers indicating the order of their 
discovery. 

Meteoric Bodies, which at times approach the Earth's 
orbit, and occasionally reach the Earth's surface. 

Comets. 

The Zodiacal Light, a ring of apparently nebulous mat- 
ter, the exact nature and position of which in the system 
are not yet determined. 

139. Explanation of the Signs. — An explanation of the 
signs by which the eight large planets are denoted, may 
enable the student to remember them more easily. 

Mercury was the messenger of the gods ; the sign of 
the planet so called ( $ ) is deduced from the outline of his 
caduceus, or rod, which was entwined by two serpents 
and surmounted by a pair of wings. Venus, the goddess 
of beauty, has for her sign a circular looking-glass with a 

large planets are there ? Name them in the order of their distances from the 
San, and make the characters hy which they are represented. How many 
asteroids are there ? How are their orbits situated ? Describe the Zodiacal 
Light. 139. Explain the meaning of the signs hy which the eight large planets 



72 THE SOLAK SYSTEM. 

handle ( 9 ). The Earth's sign is a circle, denoting its 
shape (©). Mars, the god of war, has a round shield sur- 
mounted by a spear-head (6). Jupiter's sign (If) is de- 
rived from a capital zeta (Z), the initial of his Greek name, 
Zeus. Saturn, the god of time, is represented by the scythe 
with which he mows down the human race ("£). Uranus 
is denoted by a planet suspended from the cross-bar of an 
H, the initial of Herschel, its discoverer (Ip). Neptune is 
known by his trident ( f ). 

140. Historical Details. — Of the eight large planets, 
Mercury, Venus, Mars, Jupiter, and Saturn, being visible 
to the naked eye, were known to the ancients. Uranus 
was discovered in 1781 by Sir William Herschel, from 
whom it was first commonly called Herschel. Its discov- 
erer gave it the name of Georgium Sidus, in honor of King 
George III. Both these names, however, were discarded 
for the mythological one by which it is at present known. 

141. Neptune was first seen and recognized as a planet 
by Dr. Galle, of Berlin, in 1846. The honor of its dis- 
covery is due to the French astronomer Le Verrier and 
the English Professor Adams. 

The discovery of Neptune is one of the most astonish- 
ing facts in the history of Astronomy. As we shall see in 
the sequel, every body in our system affects the motions 
of every other body ; and, after Uranus had been discov- 
ered some time, it was found that, on taking all the known 
causes into account, there was still something affecting its 
motion ; it was suggested that this something was another 
planet, more distant from the Sun than Uranus itself. The 
question was, where was this planet, if it existed. 

Adams and Le Verrier applied themselves, indepen- 
dently, to the solution of this problem, and arrived at re- 
sults which showed a remarkable agreement, the positions 

are distinguished. 140. Which of the planets were known to the ancients ? 
When and hy whom was Uranus discovered ? What other names has it had ? 
141. To whom is the honor of the discovery of Neptune due ? State the interest- 



THE SUSPECTED PLANET VULCAN. 73 

assigned the unknown planet respectively by the two 
astronomers not being a degree apart. Search was made 
in July, 1846, with the large telescope of the Cambridge 
Observatory, in the region indicated by the calculations 
of Mr. Adams ; but no planet was recognized. In the fol- 
lowing September, Le Verrier wrote to the Berlin observ- 
ers, acquainting them with the results of his investigations, 
and requesting them to explore a certain part of the heav- 
ens where he imagined the planet then to be. Thanks to 
their superior star-map (which had not yet been pub- 
lished), the planet was discovered, in accordance with 
these instructions, that same evening. 

142. The first of the asteroids, Ceres, was discovered 
in 1801 by the Sicilian astronomer Piazzi. Pallas was 
added to the list in 1802 ; Juno, in 1804 ; Yesta, in 1807 ; 
the rest have been discovered since 1844. 

143. A Suspected Planet. — Besides the eight principal 
planets mentioned above, a ninth — quite small — is sus- 
pected to exist, between Mercury and the Sun, only thir- 
teen million miles from the latter, and performing its 
revolution in about 19f days, in an orbit inclined to the 
ecliptic at an angle of 12°. A French physician, named 
Lescarbault, claimed to have discovered it crossing the Sun's 
disk in 1859. The name of Vulcan was assigned to it. 

Other observers have, at different times, seen spots of 
a planetary character rapidly cross the disk of the Sun, 
which may turn out to have been transits of Vulcan ; but 
up to the present time we can only say that the existence 
of such a planet is suspected — it is not proved. Le Ver- 
rier and other astronomers consider it not improbable, by 
reason of a certain disturbance in the motion of Mercury, 
for which a planet so situated would account. 

ing facts connected with the discovery of Neptune. 142. Which four of the 
asteroids were first discovered, and when? 143. What is said respecting a 
ninth planet, whose existence is suspected ? What appearances that have been 
observed may have been transits of Vulcan ? What seems to make the existence 

4 



74 THE SOLAR SYSTEM. 

144. Motions and Orbits of the Planets. — Let us begin 
by getting some general notions of the planetary motions 
and orbits. In the first place, all the planets travel round 
the Sun in the same direction ; and that direction, looking 
down upon the system from the northern side of it, is 
from west to east, or, in other words, in the opposite di- 
rection to that in which the hands of a clock move. Sec- 
ondly, the paths of all the planets, and of many of the 
comets, are elliptical, but some are very much more ellip- 
tical than others. 

145. Next let the student turn back to Art. Ill, in 
which we attempted to give an idea of the plane of the 
ecliptic. Now, the larger planets keep very nearly to this 
level, which is represented in the following figure : — 




The Plane of the Ecliptic and the Planetary Orbits. 



The straight line we suppose to represent the Earth's 
orbit looked at edgeways. The other lines represent the 
orbits of some of the planets and comets seen edgeways in 
the same manner. The orbits of Mars, Jupiter, Saturn, 
Uranus, and Neptune, deviate so little from the plane of 
the ecliptic, that in our figure, the scale of which is very 
small, they may be supposed to lie in that plane. With 
some of the smaller planets and comets we see the case is 
very different. The latter, especially, plunge as it were 

of such a planet probable ? 144. What motion have all the planets ? What is 
the shape of their orbits ? 145. To what plane do the orbits of the larger plan- 
eta keep very close ? Which planet's orbit has the gi-eatest flip ? How are the 



DISTANCES OF THE PLANETS. 



IS 



down into the surface of our ideal sea, or plane of the 
ecliptic, in all directions, instead of floating on it or re- 
volving in it. 

146. Moons. — Again, as we thus find planets travelling 
round the Sun, so also do we find other bodies travelling 
round some of the planets. These are called Moons, or 
Satellites. The Earth has one Moon; Jupiter has four, 
Saturn eight, Uranus four, and Neptune, according to our 
present knowledge, one. 

147. Motions of the Planets. — All the planets revolve 
round the Sun and rotate on their axes in the same direc- 
tion, i. e., from west to east. The satellites also revolve 
round their primaries in the same direction, except those 
of Uranus and Neptune, which move from east to west. 

148. Distances from the Sun. — Let us next inquire into 
the various distances of the planets from the Sun, bearing 
in mind that, as the orbits are elliptical, the planets are 
sometimes nearer to the Sun than at other times. The 
mean distances from the Sun, and the times of revolution, 
expressed in the Earth's days, are as follows : — 





Distance from the 
Sun in miles. 


Period of revolutioi 
the Sun. 

D. H. 


round 

M. 


Mercury, . 


35,393,000 . 


87 


23 


15 


Venus, . . 


66,131,000 . 


224 


16 


48 


Earth, . . 


91,430,000 . 


365 


6 


9 


Mars, . . 


139,312,000 . 


686 


23 


31 


Jupiter, 


475,693,000 . 


4332 


14 


2 


Saturn, . . . 


872,135,000 . 


10759 


5 


16 


Uranus, 


1,753,851,000 . 


30686 


17 


21 


Neptune, . 


2,746,271,000 . 


60126 


17 


20 


149. The apps 


irent size of an obje 


ct varies i 


ttdth its dis- 



orbits of the comets inclined, as regards the plane of the ecliptic ? 146. What 
are Moons ? What planets have moons, and how many has each ? 147. What 
other motion besides that in their orbits have the planets ? In what direction 
do the satellites revolve ? 148. How far is the nearest planet from the Sun ? How 
far is the farthest planet ? What is the length of Mercury's year ? Of Jupiter's ? 



76 THE SOLAR SYSTEM. 

tance ; hence the solar disk must vary in size, as seen from 
the different planets, appearing largest to Mercury, which 
is nearest to it. Fig. 31 shows the relative size of the disk 
as seen from the several planets. It is well to remember 
that the relative size of the disk, as thus shown, represents 
also the relative amount of light and heat which the plan- 
ets receive. 

150. Comparative Size of the Planets. — The equatorial 
diameters of the planets are as follows : — 



Mercury, . 
Venus, 
Earth, . 


Diameter in Miles. 
. . . 2,962 
. . . 7,510 
. . . 7,926 


Jupiter, . 
Saturn, . 
Uranus, . 


Diameter in Miles. 
. . . 85,390 
. . . 71,904 
. . . 33,024 


Mars, . . 


. . . 4',920 


Neptune, 


. . . 36,620 



151. We have before attempted to give an idea of the 
comparative size of the Earth and Sun, and of the distance 
between them ; let us now complete the picture, with the 
aid of Sir John HerschePs familiar illustration. Taking a 
globe two feet in diameter to represent the Sun, Mercury 
would be a grain of mustard-seed, revolving in a circle 
164 feet in diameter; Venus, a pea, in a circle 284 feet in 
diameter ; the Earth, also a pea, at a distance of 430 feet ; 
Mars, a rather large pin's head, in a circle of 654 feet ; the 
asteroids, grains of sand, in orbits of from 1,000 to 1,200 
feet ; Jupiter, a moderate-sized orange, in a circle nearly 
half a mile across ; Saturn, a small orange, in a circle of 
four-fifths of a mile ; Uranus, a full-sized cherry, or small 
plum, in a circle more than a mile and a half across ; and 
Neptune, a good-sized plum, in a circle about two miles 
and a half in diameter. 

Fig. 32 will help to give an idea of the relative size of 

Of Neptune's? 149. On what does the apparent size of an ohject depend? To 
which planet does the Sun look largest ? To which, smallest ? What does the 
relative size of the Sun's disk also represent ? 150. Which planet has the great- 
est diameter? Which, the smallest? How does the Earth's diameter compare 
with that of Venus ? With that of Jupiter ? 151. Give Sir John Herschel's illus- 
tration- of the comparative sizes and distances of the planets. What does Fig. 32 



SIZE OF THE SUN'S DISK. 



11 




Fig. 31.— The Kelative Size of the Sun, as seen jfkom the Planets. 



78 THE SOLAR SYSTEM. 

the Sun and planets. The black circle represents the disk 
of the Sun. The disks of the several planets are repre- 
sented by the white circles, on the same scale as that of 
the Sun, commencing with Mercury at the right of the 
upper line. 



K 




■■Hi 



IBBH 



^:^Mm~^-? 



Fig. 32.— Kelative Size of the Sun and Planets. 

152. Distances and Revolutions of the Satellites. — The 
satellites revolve round their primaries, like the planets 
round the Sun, at different distances. Our solitary Moon 
courses round the Earth at a distance of 240,000 miles, 
and its journey is performed in a month. The first satellite 
of Saturn is only about one-half of this distance from its 
primary, and its journey is performed in less than a day. 

represent? 152. What motion have the satellites ? State the distances of some 
of the satellites from their primaries, and their times of revolution. 153, Which 



THE SATELLITES. 79 

The first satellite of Uranus is about equally near, and 
requires about two and a half days. The first satellite of 
Jupiter is about the same distance from that planet as our 
Moon is from us, and its revolution is accomplished in one 
and three-quarters of our days. The only satellite which 
takes a longer time to revolve round its primary than our 
Moon, is Jap'etus, the eighth satellite of Saturn. 

The diameter of the smallest planet — leaving the 
asteroids out of the question — is 2,962 miles. Among the 
satellites we have three bodies — the third and fourth 
satellites of Jupiter, and the sixth moon of Saturn — of 
greater dimensions than one of the large planets, Mer- 
cury, and about two-thirds of the size of Mars. 

The distances and sizes of all the planets and satellites are given in 
Tables II. and III. of the Appendix. 

153. The relative distances of the planets from the Sun 
were known long before their absolute distances — just as 
we might know that one place was twice or three times as 
far away as another, without knowing the exact distance 
of either. When once the distance of the Earth from the 
Sun was known, astronomers could easily find the distance 
of all the rest from the Sun, and therefore from the Earth. 
Their sizes were next determined, for we need only to 
know the distance of a body and its apparent size, or 
the angle under which we see- it, to determine its real 
dimensions. 

154. Volumes, Masses, and Densities of the Planets. — 
In the case of a planet accompanied by satellites we can 
at once determine its weight, or mass, as will be shown 
hereafter ; and when we have ascertained its weight, 
having already obtained its size or volume, we can com- 
pare the density of the materials of which it is composed 

were known first— the relative distances of the planets from the Sun, or their 
absolute distances ? When the distance of the Earth from the Sun was deter- 
mined, what followed ? What were next determined ? 154. In what case can we 
at once determine the weight of a planet ? If we know its size, what can we 



80 THE SOLAK SYSTEM. 

with those we are familiar with here ; having first also 
obtained experimentally the density of our own Earth. 

155. Let us see what this word density means. To do 
this, let us compare platinum, the heaviest metal, with 
hydrogen, the lightest gas. The gas is, in round numbers, 
a quarter of a million times lighter than the metal, and 
therefore the same number of times less dense. If we had 
two planets of exactly the same size, one composed of 
platinum and the other of hydrogen, the latter would be 
a quarter of a million times less dense than the former. 
Now, if it seems absurd to talk of a hydrogen planet, we 
must remember that if the materials of which our system, 
including the Sun, is composed, once existed as a great 
nebulous mass extending far beyond the orbit of Neptune, 
as there is reason to believe, the mass must have been 
more than 200,000,000 times less dense than hydrogen! 

156. Philosophers have found that the mean density 
of the Earth is a little more than five and a half times 
that of water ; that is, our Earth is five and a half times 
heavier than it would be if it were made up of water. 
Looking at the planets together, we find that, as a general 
rule, they increase in density as we approach the Sun, 
Mercury being the densest, Venus and Mars agreeing very 
nearly with the Earth in density, Jupiter being only \ as 
dense as the Earth, and the more distant planets, Saturn, 
Uranus, and Neptune, being still less dense than Jupiter. 

157. A table follows, showing the relative volume, 
mass, and density of the planets, the Earth's being repre- 
sented by 100. The absolute volume of the Earth being, 
in round numbers, 259,400,000,000 cubic miles, and its 
weight 6,000,000,000,000,000,000,000 tons, the volume and 
weight of the other planets can be readily found from 
this table. 

then do ? 155. Illustrate the meaning of the word density by comparing platinum 
with hydrogen. 156. How does the density of the Earth compare with that of 
water? Comparing the other planets with the Earth as regards density, what 
ilo we find ? 157. Which planet is about 300 times as heavy as the Earth ? How 



VOLUME, MASS, AND DENSITY 



81 



a 


Volume. 


Mass. 


Density. 


Mercury, . . 


5 


7 . 


. 124 


Venus, . . 


85 


79 . 


92 


Earth, . . 


100 


100 . 


100 


Mars, . . 


14 


12 . 


96 


Jupiter, . . 


. 138,743 


30,000 . 


22 


Saturn, . . 


74,689 


9,000 . 


12 


Uranus, . . 


7,236 


1,300 . 


18 


Neptune, . . 


9,866 


1,700 . 


17 


158. Summing up. 


— To sum up, then, our first gent 



survey of the Solar System, we find it composed of planets, 
satellites, comets, and several rings of meteoric bodies; 
the planets, both large and small, revolving round the Sun 
in the same direction, the satellites revolving round the 
planets. We have learned the mean distances of the 
planets from the Sun, and have compared the distances 
and times of revolution of some of the satellites. We 
have also seen that the volumes, masses, and densities 
of the planets have been determined. There is still much 
more to be learned, about both the system generally, and 
the planets particularly; but it will be best first to in- 
quire somewhat minutely into the movements and struc- 
ture of the Earth on which we dwell. 



CHAPTER V. 



THE EARTH. 



159. We took the Sun as a specimen of the stars, 
because it was the nearest star to us, and we could there- 
fore study it best ; so now let us take our Earth, with 
which we should be familiar, as a specimen of the planets. 

does Jupiter compare in density with the Earth? Which planet has the least 

density ? 158. Sum up what we have thus far stated respecting the Solar System. 

159. What hody do we first consider, as a specimen of the planets? 160. 



82 



THE EARTH. 



160. Shape of the Earth.— In the first place, tne Earth 
is round. Had we no proof, we might have guessed this, 
because both Sun and Moon, and the planets observable 
in our telescopes, are round. But we have proof. The 
Moon, when eclipsed, enters the shadow of the Earth ; and 
this shadow, as thrown on the bright Moon, is circular. 

Moreover, if we watch ships putting out to sea, we lose 
first the hull, then the lower sails, until at last the highest 




Fig. 33.— Proof of the Curvature of the Earth's Surface. 

parts of the masts disappear. So the sailor, when he 
sights land, first catches the tops of mountains, or other 
high objects, before he sees the beach or port. If the 
surface of the Earth were an extended plain, this would 



What is the shape of the Earth ? What proofs have we that the Earth is round : 



THE SENSIBLE HOKIZON. 



83 



not happen ; we should see the nearest things and the 
largest things first. As it is, every point of the Earth's 
surface is the top, as it were, of a flattened dome inter- 
posed between us and distant objects. The inequalities 
of the land render this fact much less obvious on terra 
firma than on the surface of the sea. 

Again, the roundness of the Earth has been proved by 
navigators, who, sailing in one direction, either east or west 
(as nearly as the different bodies of land would permit), 
have returned to the place from which they set out. 

161. The Sensible Horizon. — On all sides of us we see 
a circle of land, or sea, or both, on which the sky seems to 
rest ; this is called the Sensible Horizon. If we observe it 
from a little boat on the sea, or from a plain, this circle is 
small ; but if we look out from the top of a ship's mast or 
from a hill, we find it greatly enlarged — in fact, the 
higher we go the more is the horizon extended, always 
however retaining its circular form. IsTow, the sphere is 




Fig. 34.— Hokizoxs of the Same Place, at Diffekent Heights. 

the only figure which, looked at from any external point, 
is bounded by a circle ; and as the horizons of all places 
are circular, the Earth is a sphere, or nearly so. 

161. What is the Sensible Horizon ? What proof of the Earth's roundness does 



84 



THE EAETH. 




*°utm pour 
Fig. 35.— Poles and Equator. 



162. Poles, etc. — The Earth is 
not only round, but it rotates or 
turns on an axis, as a top does 
when it is spinning ; and the names 
of JSTorth Pole and South Pole are 
given to those points where the 
axis would come to the surface if 
it were a great iron rod instead of 
a mathematical line. Half-way be- 
tween these two poles, there is an 
imaginary line running round the 
Earth, called the Equator or Equinoctial Line. 

The line through the Earth's centre from pole to pole, 
is called the Polar Diameter ; the line through the Earth's 
centre from any point of the equator to the opposite point, 
is called the Equatorial Diameter • and one of these, as 
we shall see, is longer than the other. 

163. Proofs of the Earth's Rotation. — We owe to the 
ingenuity of the French philosopher, Foucault, two ex- 
periments which render the Earth's rotation visible to the 
eye. For, although it is made evident by the apparent 
motion of the heavenly bodies and the consequent suc- 
cession of day and night, we must not forget that these 
effects might be, and for long ages were thought to be, 
produced by a real motion of the Sun and stars round the 
Earth. 

1 64. The first experiment consists in allowing a heavy 
weight, suspended by a fine thread or wire, to swing back- 
ward and forward like the pendulum of a clock. Now, 
if we move the beam or other object to which such a 
pendulum is suspended, we shall not alter the direction in 
which the pendulum swings, as it is easier for the thread 



the sensible horizon afford ? 162. What is meant by the North and the South 
Pole of the Earth? By the Equator? By the Polar Diameter? By the Equa- 
torial Diameter ? Which of these two diameters is the longer ? 163. To whom 
are we indebted for having made the Earth's rotation on its axis visible to the 
eye ? 164. Give an account of the first experiment. Where and how might such 



PKOOFS OF THE EAETH'S EOTATION. go 

which supports the weight to twist than for the heavy- 
weight itself to alter its course when once in motion in 
any particular direction. Therefore, if the Earth were at 
rest, the swing of the pendulum would always be in the 
same direction with regard to the support and the sur- 
rounding objects. 

Foucault's pendulum was suspended from the dome of 
the Pantheon in Paris, and a fine point at the bottom of 
the weight was made to leave a mark in sand at each 
swing. The marks successively made in the sand showed 
that the plane of oscillation varied with regard to the 
building. Here, then, was a proof that the building, and 
therefore the Earth, moved. 

Such a pendulum swinging at either pole would make 
a complete revolution in 24 hours, and would serve the 
purpose of a clock were a dial placed below it with the 
hours marked. As the Earth rotates at the north pole 
from west to east, the dial would appear to a spectator, 
carried round like it by the Earth, to move under the 
pendulum from west to east, while at the south pole the 
Earth and dial would travel from east to west ; midway 
between the poles, that is, at the equator, this effect, of 
course, is not noticed, as there the two motions in opposite 
directions meet. 

165. The second experiment is based upon the fact 
that, when a body turns on a perfectly true and symmetrical 
axis, and is left to itself in such a manner that gravity is 
not brought into play, the axis maintains an invariable 
position ; indeed, to maintain its position, it will even over- 
come slight obstacles. If, then, the axis of a heavy disk, 
so freely suspended that it is at almost entire liberty to 
turn in any direction, be made to point to a star, which is 
a thing outside the Earth, it will continue to point to it — 

a pendulum "be made to serve as a clock ? How would the dial appear to move 
at the north pole ? How, at the south pole ? How, at the equator ? 165. On 
what fact is the second experiment based ? 166. What instrument does it em- 



86 



THE EAETH. 



sV 



even turning, if the Earth's rotation makes it necessary, in 
order to keep the same absolute direction. 

166. The Gyroscope is an instrument so made that a 
heavy disk, freely suspended and set in very rapid motion, 
shall be able to rotate for a long period, and that all dis- 
turbing influences, the action of gravity among them, may 
be as far as possible prevented. 

Now, if the Earth were at rest, there would be no 
apparent change in the position of the axis, however long 
the wheel might continue to turn ; but if the Earth moves 
and the axis remains at rest, there should be some dif- 
ference. Experiment proves that there is a difference, and 
just such a difference as is accounted for by the Earth's 
rotation. In fact, if we so arrange the gyroscope that the 
axis of its rotation points to a star, it will remain at rest 
with regard to the star, while it varies 
with regard to surrounding objects on 
the Earth. This is proof positive that 
it is the Earth which rotates on its axis, 
and not the stars that revolve round 
it ; for in the latter case the axis of the 
gyroscope would remain invariable with 
regard to the Earth, and change its di- 
rection with regard to the star. 

Fig. 36 represents the interesting instrument 
with which the experiment just referred to is 
made. D is a heavy symmetrical metallic disk, 
mounted on an axis which passes through 0, the 
centre of the disk, and is perpendicular to its two 
sides. This axis terminates in pivots C 0", which 
fit into holes made at opposite extremities of the 
diameter of a circular ring B B\ which is furnished 
with two knife-edges (like those of a balance), 
and so arranged that BB' is the diameter of the 
ring perpendicular to C C. The knife-edges rest 
in holes made at opposite extremities of the horizontal diameter of a ver- 




Fig. 36.— The Gyro- 
scope. 



ploy? How does the gyroscope prove the Earth's rotation? Describe the con- 



PAEALLELS AND MEEIDIANS. 



87 



tical circle A A', which is suspended by a fine wire from the fixed point S. 
At A' is a pivot, which rests in a small hole. All the pivots are highly 
polished, so that friction may be avoided as much as possible; and the 
different parts »re so adjusted that is the common centre of the disk 
and the rings. The axis C C may be made to point in' any direction by 
moving first the ring A A', and then the ring BB, into proper positions. 

To perform the experiment, BB is removed from its supports, and, 
the disk having been made to revolve rapidly, is then restored to its 
place. Whatever star C C is directed toward, it continues to point to 
as long as the disk rotates, and thus, as stated above, changes its position 
relatively to objects on the Earth ; unless, indeed, the star be the polar 
star, in which case no change of direction will be observed. 

167. Imaginary Lines on 
the Earth's Surface. — If we 

look at a terrestrial globe, we 
find that the equator is not 
the only line marked upon it. 
There are small circles paral- 
lel to the equator, called Paral- 
lels; and large circles, called 
Meridians, passing through 
both poles, and dividing the 

Fig. 37.— parallels and Meridians, equator into equal parts. 

These lines are for the purpose of determining the exact 

position of a place upon the globe. 

168. Latitude. — The distance of any place from the 
equator, measured in degrees (or 360ths) of its meridian, 
is called its Latitude. If north of the equator, it is said 
to be in north latitude; if south of the equator, in south 
latitude. As either pole is 90° distant from the equator, 
the greatest latitude a place can have is 90°. 

169. Longitude. — But something else besides latitude 
is needed to define the position of a place. Accordingly, 
some meridian is taken, — in this country either the merid- 

struction of the gyroscope. What is done to the instrument when the experi- 
ment is performed ? 167. What circles do we find on a terrestrial globe ? What 
is their object? 168. What is Latitude ? What is the difference between North 
and South Latitude ? What is the greatest latitude a place can have ? 169. What 
else besides latitude is needed to define the position of a place ? What is Longi- 




88 THE EAKTH. 

ian of Washington, or that which passes through Green- 
wich, near London, where the principal observatory of 
England is situated ; and the distance of the place from 
this First Meridian, as it is called, measured in degrees 
(or 360ths of its parallel), determines, with its latitude, its 
exact position. Distance from the first meridian, so meas- 
ured, is called Longitude. Places east of the first merid- 
ian are said to be in east longitude, and those west of the 
first meridian in west longitude. As the distance half 
round the Earth is 1 80°, the greatest longitude a place 
can have is 180°. 

-£.:Fri<!iazo„ e l 7°- Zones. — On the terres- 

trial globe we find parallels of 



S/^ — . Arctic! C ircle _J>V 

4 



latitude and meridians of longi- 




^icojfCancer ^A tude laid d ° WI1 10 ° 0Y 15 ° &V art ' 

*[..-- 'i " ..W Besides these, 23-J° from the 

^\_^^^^r77^^j S equator on either side are the 

jv opic off Capricorn — :] Tropics, — the Tropic of Cancer 

^X^jmt^^c^^y^ north of the equator, the Tropic 

of Capricorn south of it. 
™ o m n * l t ° ri At the same distance from 

Fig. 38.— The Polab Circles, 

teopics, and zones. the poles are the Polar Circles, 

the northern one being distinguished as the Arctic Circle, 
the southern as the Antarctic Circle. The tropics and 
polar circles divide the Earth's surface into five belts, or 
Zones — one torrid, two temperate, and two frigid zones, as 
shown in Fig. 38. 

171. Polar and Equatorial Diameter, — The distance 
along the axis of rotation, from pole to pole, through the 
Earth's centre, is shorter than the distance through the 
Earth's centre from any point of the equator to the op- 

tude? What meridian is generally taken as the First Meridian? What is the 
difference between East and West Longitude ? What is the greatest longitude a 
place can have ? 170. What are found 23^ degrees from the equator ? What 
circles lie 23>£ degrees from the poles ? Into what do the tropics and polar cir- 
cles divide the Earth's surface ? How many degrees wide is each frigid zone ? 
How wide is the torrid zone ? How wide is each temperate zone ? 171. What 



THE EAETH'S DIAMETER. 



89 



posite one. In other words, the Polar Diameter (Art. 
162) is shorter than the Equatorial Diameter. Then- 
lengths are as follows : — 

Feet. Miles. 

Mean Equatorial Diameter, . 41,848,380 . 7,925ff§ 

Polar Diameter, 41,708,710 . 7,899^ff 

Difference in length, about 26^- miles. 

This difference is but small ; yet it proves that the Earth 
is not a sphere, but an oblate spheroid (see Art. 39). 

172. The mean equatorial diameter is given above, for 
it is found that the equatorial circumference is not a per- 
fect circle, but an ellipse, the difference between the major 
and minor axis of which is more than If miles. The 
equatorial diameter which runs from longitude 14° 23' 




Fig. 



-Circle and Ellipses. GIT, a circle. I J, LM, ellipses of different 
eccentricities. F, C, foci of L M. £>, E, foci of IJ. 



is meant by the Polar Diameter of the Earth ? The Equatorial Diameter ? How do 
they compare in length '? What, then, is the form of the Earth ? 172. Why is the ex- 
pression mean equatorial diameter used ? 173. What produces the succession of day 



90 THE EAKTH. 

east of Greenwich to 165° 37' west is over If miles longer 
than the one at right angles to it. 

173. Motions of the Earth. — The Earth turns on its 
axis, or polar diameter, in 23h. 56m. In this time we get 
the succession of day and nighty which is due to the 
Earth's rotation. The Earth also goes round the Sun, and 
the time in which that revolution is effected we call a year. 

174. Revolution round the Sun. — The Earth and all 
the other planets move round the Sun in elliptical orbits. 
An ellipse was denned in Art. 35 ; its shape depends on 
the distance between its foci. It may be a decided oval, 
like L M in Fig. 39; or, if the foci are near together, it 
may have so little eccentricity as to be indistinguishable 
from a circle, as in the case of I J, which looks as if it 
were parallel to the circle G H. The planetary orbits 
differ but little from circles. 

175. Perihelion and Aphelion. — The Sun is not at the 
centre of the ellipses described by the planets, but at one 
of the foci. Hence every planet is nearer the Sun at one 
time of its revolution than at another. When nearest to 
the Sun, a planet is said to be in perihelion (from the 
Greek words nepi, near, and r\Xiog, the Sun) ; when farthest 
off, in aphelion (dnb, from, and 77/Uoc, the Sun). 

176. The eccentricity of the ellipse described by the 
Earth is only ^ so that when the orbit is represented on a 
small scale, as in Fig. 40, no deviation from a circle is per- 
ceptible. The Earth is 3,000,000 miles nearer the Sun in 
perihelion (at P, Fig. 40) than in aphelion (at A). 

The Earth is in perihelion at present about January 
1st, the time of the southern summer, and in aphelion 
about July 1st, the period of the northern summer. This 

and night? What other motion has the Earth? 174. What is the shape of all 
the planetary orbits ? How may ellipses differ? What kind of ellipses are the 
planetary orbits ? 175. How is the Sun situated, as regards these ellipses ? 
When is a planet said to be in perihelion, and when in aphelion? 176. What is 
the difference in the Earth's distance from the Sun at these two points? At 
what time of the year is the Earth in perihelion, and at what in aphelion? 



PERIHELION AND APHELION. 



91 




greater near- 
ness to the 
Sun intensifies 
the heat of the 
southern sum- 
mer, and ac- 
counts for the 
fact that the 
temperature of 
this season is 
higher in Aus- 
tralia and 
Southern Afri- 
ca than in cor- 
responding lat- 
itudes north Fig. 40.— The Eakth's Okbit. S, the Sun ; P, the Earth 
of the eaua- in perihelion ; J., the Earth in aphelion. 

tor. About 3,600 years before the creation of Adam, the 
Earth was nearest to the Sun during the summer of the 
northern hemisphere, and farthest off in the northern win- 
ter; which must have made the northern su mm er much 
hotter than it now is (according to Sir John Herschel, 23°), 
and the northern winter as much colder. 

177. Velocity of the Earth's Motions, — Let us now in- 
quire with what velocity the two motions of the Earth are 
performed. 

As regards the diurnal motion, or rotation on the axis, 
it is clear that all the points on any meridian must make a 
complete revolution in the same time, while the circles or 
distances traversed in making such revolution diminish as 
we go from the equator to either pole. Hence, there is a 
material difference in the velocity of points in different 
latitudes. The poles have no rotary motion at all, and 



What is the consequence, as regards the southern summer-? When must the 
northern summer have heen hotter than it now is, and why? 177. Show why- 
different parts of the Earth's surface have a different velocity of rotation. What 



92 THE EARTH. 

the regions about them very little. Points in the latitude 
of Paris have a velocity of about 330 yards a second; 
those in the latitude of Washington, about 375 yards; 
and those on the equator, about 507 yards. It has been 
demonstrated that the time of rotation has not varied one- 
hundredth of a second during the last two thousand years. 

178. The velocity of the Earth in its orbit is constantly 
varying, being greatest when the Earth is in perihelion, as 
the Sun's attraction is then strongest. Its average rate is 
about 19 miles a second— more than a thousand times 
greater than that of the fastest locomotive. Two philos- 
ophers, who have attempted to determine the amount of 
heat that would be developed by the abrupt stoppage of 
the Earth in its orbit, tell us that it would suffice to melt 
the entire globe and reduce the greater part of it to vapor. 

179. The reason why we are unconscious of moving 
with this immense rapidity is that we have never known 
any other condition, and that the whole bulk of the Earth 
and every object on and around it, including the atmos- 
phere and clouds, participate in the motion. 

180. Inclination of the Earth's Axis. — Now refer to 
Art. 112, in which we spoke of the position of the Sun's 
axis. We found that the Sun was not floating uprightly 
in our sea, the plane of the ecliptic ; it was dipped down 
in a particular direction. So it is with our Earth. The 
Earth's axis is inclined in the same manner, but to a much 
greater extent (23° 27' 24"). The direction of the inclina- 
tion, as in the case of the Sun, is always the same. 

181. Effects of the Earth's Motions. — We have, then, 
two completely distinct motions — one performed in a day, 
round the axis of rotation, which, so to speak, remains 

is the velocity in the latitude of Paris ? In that of Washington ? At the equa- 
tor ? What has heen shown respecting the time of rotation ? 178. What causes 
constant changes in the velocity of the Earth as it revolves round the Sun ? What 
is the average rate of the Earth's motion in its orbit ? 179. Why are we uncon- 
scious of this rapid motion ? 180. State the facts respecting the inclination of 
the Earth's axis. 181. How many motions, then, has the Earth, and what do we 



EFFECTS OF THE EARTH'S MOTIONS. 



93 



parallel to itself; the other round the Sun, performed in a 
year. To the former motion we owe the succession of day 
and night ; to the latter, combined with the inclination of 
the Earth's axis, we owe the seasons. 



Sols tW 


AUTUMNAL 




^m^^ f 8^€k "" 




■-§BKI- w 

WSmm 

^^^^P^iS "-•.■■:-.■.'■■■■■"■■.-. ■■-.;■■ 


LION I 




^- ' " - c - 


Us 


' VERNAL 
EQUINOX 



Fig. 41.— Position of the Earth in its Orbit at Different Seasons. 

182. Succession of Day and Night. — Fig. 41 represents 
the orbit of the Earth, with the Sun at its centre. It also 
shows how the axis of the Earjh is inclined, its direction 
being toward the Sun on the 21st of June, and the inclina- 
tion being about 23^°. Now, if we bear in mind that the 
Earth is spinning round once in twenty-four hours, we 
shall immediately see how it is we get day and night. 
The Sun can only light up that half of the Earth turned 
toward it ; consequently, at any moment, one-half of our 
planet is in sunshine, the other in shade — the rotation of 
the Earth bringing each part in succession from sunshine 
to shade, and from shade to sunshine. 

owe to each ? 182. What does Fig. 41 represent ? How is it that we get the sue- 



04 THE EAETH. 

183. But it will be asked, "How is it that the days 
and nights are not always equal?" We answer, by 
reason of the inclination of the axis. 

In the first place, the days and nights are equal all 
over the world on the 22d of March and the 2 2d of 
September, which dates are called the vernal and the 
autumnal equinox for that very reason — equinox being 
derived from two Latin words meaning equal night. 

ISTow let us look at the small circle marked on the 
Earth — it is the arctic circle ; and let us suppose ourselves 
living in Greenland, just within that circle. What will 
happen? At the vernal equinox (it will be most con- 
venient to follow the order of the year) we find that circle 
half in light and half in shade. One-half of the twenty- 
four-hours (the time of one rotation), therefore, will be 
spent in sunshine, the other in shade : in other words, the 
day and night will be equal, as before stated. Gradually, 
however, as we approach the summer solstice (going from 
left to right), we find the circle coming more and more 
into the light, in consequence of the inclination of the 
axis, until, when we arrive at the solstice, in spite of the 
Earth's rotation we cannot get out of the light. At this 
time we see the midnight sun due north. The Sun, in 
fact, does not set. 

The solstice passed, we approach the autumnal equinox, 
when again we shall find* the day and night equal, as we 
did at the vernal equinox. But when we come to the 
winter solstice, we get no more midnight suns : as shown 
in the figure, all the circle is situated in the shaded 
portion ; hence, in spite of the Earth's rotation, we cannot 
get out of the darkness, and we do not see the Sun even 
at noonday. 

There will now be no difficulty in understanding how 
at the poles the years consist of one day of six months' 

cession of day and night ? 183. Explain the inequality of the days and nights, 
and the changes that occur in this respect as the Earth advances in her orhit. 



LENGTH OF DAY AND NIGHT. 



95 



duration, and one night of equal length. To comprehend 
our long summer days and short nights, we have only to 
take a point about half-way between the arctic circle and 
the equator, as marked on the plate, and reason in the 
same way as we did for Greenland. At the equator we 
shall find the day and night always equal. 

1 84. Here is a table showing the length of the longest 
day in different latitudes, from the equator to the poles : — 






/ 








16 


44 


30 


48 


41 


24 


49 


2 


54 


31 


58 


27 


61 


19 


63 


23 


64 


50 



(Equator) 



Hours. 

. 12 

. 13 

. 14 

. 15 

. 16 

. 17 

. 18 

. 19 

. 20 

. 21 



o 

65 
66 
66 



48 
21 
32 



Hours. 
. 22 
. 23 
. 24 



Months. 

. 1 

. 2 

. 3 

. 4 

. 5 

. 6 



67 23 ... . 

69 51 ... . 

73 40 . . . . 

78 11 ... . 

84 5 . . . . 

90 (Pole) . 

185. What we have said about the northern hemisphere 
applies equally to the southern, but the diagram will not 
hold good, as the northern winter is the southern summer, 
and so on ; moreover, if we could look upon our Earth's 
orbit from the other side, the direction of the motions 
would be reversed. The pupil should construct a diagram 
for the southern hemisphere for himself. 

186. The Change of Seasons. — The changes to which 
we inhabitants of the temperate zones are accustomed, the 
heat of summer, the cold of winter, the medium tempera- 
tures of spring and autumn, depend simply upon the 
height which the Sun attains at mid-day — for the more 
nearly perpendicular the Sun's rays are, the more heat 
does the Earth absorb from them. This is proved by the 

184. How long are the days and nights at the poles ? At the equator ? In what 
latitude is the length of the longest day 15 hours ? Twenty hours ? One 
month ? Three months ? 18G. On what do the changes of season in the temperate 



96 THE EARTH. 

facts that on the equator the Sun is never far from the 
zenith — that is, the point directly overhead — and we have 
perpetual summer : near the poles, the Sun never gets 
very high, and we have perpetually the cold of winter. 
How, then, are the changing seasons in the temperate 
zones caused? 

187. In Fig. 41 we were supposed to be looking down 
upon our system. We will now take a section from 
solstice to solstice through the Sun, in order that we may 
have a side view of it. Here, then, in Fig. 42, we have 
the Earth in two positions, and the Sun in the middle. 




Fig. 42. — Explanation of the apparent Altitude op the Sun in Summer 
and Winter. 

On the left we have the Earth at the winter solstice, when 
the axis of rotation is inclined away from the Sun to the 
greatest possible extent. On the right we have it at the 
summer solstice, when the axis of rotation is inclined 
toward the Sun to the greatest possible extent. The line 
a b in both represents a parallel of latitude in the north 
temperate zone. The dotted line from the centre through 
b in the figure on the left, and through a in that on the 
right, shows the direction of the zenith — the direction in 
which our body points when we stand upright. We see 
that this line forms a larger angle with the line leading to 
the Sun — that is, the two lines open out wider — at the 
winter, than they do at the summer, solstice. Hence in 

zones depend? How is this proved? 187. With Fig. 42, show that the Sun 
attains different heights at different seasons. How is the Earth's axis inclined 



THE SEASONS, HOW PRODUCED. 97 

the latitude indicated the Sun is seen in winter at noon, 
low down, far from the zenith, while in summer it is 
nearly overhead. 

The pupil should now make a similar diagram, to 
represent the position of the Sun at the equinoxes ; he will 
find that the axis is not then inclined either to or from 
the Sun, but sideways, — the result being that the Sun 
itself is seen at the same distance from the point overhead 
in spring and autumn. Hence the temperature is nearly 
the same, though Nature apparently works very differently 
at these two seasons; in one we have seed-time, in the 
other the fall of the leaf. 



1 




™;:' : |g|j 


BF^fx x"' N NX^H 


^R=SSBW=p!T 


pp \ \ X \ N -^^B 


* - ff ■ 


1 v ^%JIf'"- 

j—--\ SPiM 

B—M 1 


fcfiffijSSl 



Fig. 43.— The Earth, as seen from the Sun at the Summer Solstice 
(Noon at London). 

188. Perhaps the Sun's action on the Earth, in giving 
rise to the seasons, may be made clearer by inquiring how 
the Earth is presented to the Sun at the four seasons — 
that is, how the Earth would be seen by an observer at 

at the equinoxes ? What is the consequence, as regards the temperature? 188. 
What do Figs. 43 and 44 represent ? What is the difference in the situation of 

5 



98 THE EAETII. 

the Sun. First, then, for summer and winter. Figs. 43 
and 44 represent the Earth as it would be seen from the 
Sun at noon in London, at the summer and winter solstices. 
In the former, England is seen well down toward the 
centre of the disk, where the Sun is vertical, or overhead ; 
its rays are therefore most felt, and summer prevails. In 
the latter, England is near the northern edge of the disk, 




Fig. 44.— The Earth, as seen from the Sun at the Winter Solstice 
(Noon at London). 

and farthest from the region where the Sun is overhead ; 
the Sun's rays are consequently feeble, and winter reigns. 
189. In Figs. 45 and 46, representing the Earth at the 
two equinoxes, we see that the position of England, with 
regard to the centre of the disk, is the same — the only 
difference being that in the two figures the Earth's axis is 
inclined in different directions. Hence, there is no differ- 
ence of temperature at these periods. 

places in northern latitudes, in the two diagrams ? 189. What do Figs. 45 
and 46 represent ? What is the only difference noticeable in these diagrams? 



DIFFERENCE OF TIME. 



09 




Fig. 45. 



-The Earth, as seen ebom the Sun at the Autumnal Equinox 
(Noon at London). 



190. Figs. 43, 44, 45, and 46, all represent London on 
the meridian which passes through the centre of the 
illuminated side of the Earth. It must therefore be noon 
at that place, as noon is half-way between sunrise and 
sunset. All the places represented on the western border 
have the Sun rising upon them; all the places on the 
eastern border have the Sun setting. As, therefore, at 
the same moment of absolute time we have the Sun rising 
at some places, overhead at others, and setting at others, 
we cannot have the same time, as measured by the Sun, 
at all places alike. 

191. Difference of Time and Longitude. — In fact, as 
the Earth, whose circumference is divided into 360°, turns 
round once in twenty-four hours, the Sun appears to travel 
one twenty-fourth of 360°, or 15°, in one hour, from east to 
west. One degree of longitude, therefore, makes a differ- 



190. How clo these figures show that the time, as measured hy the Sun, differs at 
different places ? 191. What difference of time does one degree of longitude 



100 



THE EARTH. 




Fig. 46. — The Earth, as seen from the Sun at the Vernal Equinox 
(Noon at London). 

erne of four minutes of time, and vice versa, — the more 
easterly longitude having the later time. 

The difference of longitude between New York and London being 
about 14°, the difference of time is 4 times 74 minutes, or 4h. 56m.— the 
time of London being later, because, being east of New York, the Sun 
comes sooner to its meridian. When, therefore, it is noon at New York, 
it is 56 minutes past 4 p. m. at London. 

When it is noon at San Francisco, it is about 5 minutes after 3 p. m. 
at Philadelphia ; required, their difference of longitude. Their difference 
of time being 3h. 5m., or 185 minutes, their difference of longitude will 
be as many times 1° as 4 minutes are contained times in 185 minutes, 
or 46i°. 

By this easy process navigators determine their longitude at sea. 
Taking with them a chronometer (an accurate watch) set according to the 
time of a given place (as, Greenwich or Washington), they ascertain the 
local time by observing with the sextant when the sun is at its high- 
make? Why is this so? Of two places in different longitudes, which has the 
later time ? When it is noon at New York, it is about 56 minutes past 4 p. m. at 
London ; what is their difference of longitude ? The difference of longitude be- 
tween San Francisco and Philadelphia being about 46^°, when it is noon at 
Philadelphia what time is it at San Francisco ? How do navigators determine 



STEUCTUEE OF THE EAETH. i l 

est point ; it is then noon. Reducing the difference of time to difference 
of longitude, as above, they find that they are so many degrees east or 
west of the meridian of the place for which their chronometer is set. 

192. Structure of the Earth. — Having said so much of 
the motions of our Earth, let us now turn to its structure, 
or physical constitution. 

We are all acquainted with the present appearance of 
our globe ; we know that its surface is here land, there 
water ; and that the land is, for the most part, covered 
with soil which permits of vegetation, varying according 
to the climate ; while in some places meadows and wood- 
clad slopes give way to rugged mountains, which rear 
their bare or ice-clad peaks to heaven. 

The first question that arises is, Was the Earth always 
as it is at present ? The answer given by Geology and 
Physical Geography is, that the Earth was not always as 
we now see it, and that changes have been going on for 
millions of years, and are going on still. 

193. The Earth's Crust. — It has been found that what 
is called the Earth's crust — that is, the outside of the 
Earth, as the peel is the outside of an orange — is composed 
of various rocks of different kinds and ages, all of them, 
however, belonging to two great classes : — 

Class I. Rocks that have been deposited by water : these 
are called Stratified or Sedimentary Rocks. 

Class II. Rocks that once were molten : these are called 
Igneous Rocks. 

194. Stratified Rocks. — The stratified rocks have not 
always existed, for when we come to examine them closely 
it is found that they are piled one upon another in succes- 
sive layers, as shown at the right of Fig. 48 below — the 
newer rocks lying on the older ones. The order in which 

their longitude at sea ? 192. Describe the preset^ appearance of our globe. Was 
it always thus ? 193. Of what is the Earth's crust composed ? Into what classes 
are Rocks divided ? 194. How are the Stratified Rocks arranged ? Give a list 



102 



THE EAETH. 



these rocks have been deposited, beginning with the up- 
permost, or those of latest formation, is as follows : — 



Cainozoic, or Tertiary 



Upper 

Lower 
f Upper 
Mesozoic, or Secondary : < t ower 



Palaeozoic, or Primary : 



Upper 



Lower 



( Alluvium. 
\ Drift. 
( Crag. 
Eocene. 

Cretaceous. 
Oolite. 
Lias. 
Trias. 

Permian. 

Carboniferous. 

Devonian. 

Silurian. 

Cambrian. 

Laurentian. 



195. That these beds have been deposited by water, 
and principally by the sea, is proved by two facts : First, 
that in their formation they resemble the beds being de- 
posited by water at 
the present time; ||IIU1 ? 
Secondly, that they U 
nearly all contain the f ||| 
remains of fishes, rep- 
tiles, and shell-fish, 
in great abundance — 
indeed, some of the 
beds are composed 
almost entirely of 
the remains of ani- 
mal life. 

Such remains, be- FlG - 47.— Fossil Fish. 

ing dug out of the Earth, are called Fossils (from the Latin 

of the stratified rocks in order, beginning with the latest. 195. How is it proved 
that these rocks have been deposited by water? What other name has been 
given to the stratified or sedimentary rocks ? Why ? What are Fossils ? 196. 




STKATIFIED KOCKS. 



103 



fossilis, dug). From their containing fossils, the stratified 
rocks have been called Fossiliferous. 

1 96. It must not be supposed that the stratified rocks of 
which we have spoken, are everywhere met with as they 
are shown in the table. Each bed could have been depos- 
ited only on those parts of the Earth's crust which were 
under water at the time ; and since the earliest period of 
the Earth's history, volcanic action, earthquakes, and 
changes of level have been at work, as they are now — and 
much more effectively, either because the changes were 
more decided and sudden, or because they extended over 
immense periods of time. 

It is found, indeed, that the stratified rocks have been 
upheaved and worn away again, bent and twisted to an 
enormous extent. Instead of being horizontal, as they 
must have been when they were originally formed at the 
bottom of the sea, they are now found in some cases tilted, 
as at the left of Fig. 48, and in others bent into irregular 
curves, as in Fig. 49. 




Fig. 48.— Stratified Rocks, tilted and 
horizontal. 




Fig. 49.— Stratified Rocks, 
curved. 



Had this not been the case, the mineral riches of the 
Earth would forever have been out of our reach, and the 
surface of the Earth would have been a monotonous plain. 
As it is, although it has been estimated that the thickness 
of the series of stratified rocks, if found complete in any 



What has interfered, in places, with the original arrangement of the stratified 
rocks ? How are they sometimes found? What would be the thickness of the 
stratified rocks, if complete in one locality ? What do we find with respect to 
each member of the series ? What advantage results from this tilting ? 197. How 



104 THE EAETH. 

one locality, would be 14 miles, each member of the series 
is found at the surface at some place or other. 

197. Igneous Rocks. — The whole series of sedimentary- 
rocks, from the most ancient to the most modern, have 
been disturbed by eruptions of volcanic materials, similar 
to those thrown up by Vesuvius and other volcanoes ac- 
tive in our own time, and intrusions of rocks of igneous 
origin from below. Of these igneous rocks, granite, which 
in consequence of its great hardness is largely used for 
paving and macadamizing, may here be taken as an exam- 
ple. These rocks are easily distinguishable from those of 
the first class, as they have no appearance of stratification 
and contain no fossils ; their constituents are different and 
are irregularly distributed throughout the mass. 

If we strip the Earth, then, in imagination, of the sedi- 
mentary rocks, we come to a kernel of rock, the constitu- 
ents of which it is impossible to determine. It may, how- 
ever, be supposed to be analogous to the older rocks of 
the granitic series, and to have been part of the original 
molten sphere, which must have been both hot and lumi- 
nous, in the same way that molten iron is both hot and 
luminous. Doubtless there was a time when the surface 
of our Earth was as hot and luminous as the surfaces of 
the Sun and stars are still. 

198. The Interior of the Earth. — Now, suppose we 
have a red-hot cannon-ball; what happens? The ball 
gradually parts with, or radiates away, its heat, and as it 
cools it ceases to give out light ; but its centre remains 
hot long after the surface in contact with the air has 
cooled. 

So precisely has it been with our Earth. We have 
numerous proofs that the interior of the Earth is at a high 

have the whole series of stratified rocks been disturbed ? What may be taken as 
an example of the Igneous Kocks? How are the igneous rocks distinguishable 
from the sedimentary? If we could strip the Earth of the sedimentary rocks, 
what should we come to? What was once doubtless the case respecting the 
Earth's surface ? 198. What is the condition of the interior of the Earth ? What 



INTERIOR OF THE EAETH. 10 5 

temperature at present, although, its surface has cooled 
down. Our deepest mines are so hot that, without a per- 
petual current of cold fresh air, it would be impossible for 
the miners to live in them. The water brought up in 
artesian wells is found to increase in temperature 1 degree 
for from 50 to 55 feet of depth. Again, there are hot 
springs coming from great depths, the water of which is, 
in some cases, at the boiling-point — that is, 212° of Fahr- 
enheit's thermometer. In the hot lava emitted from vol- 
canoes we have further evidence of this interior heat, and 
that it is independent of the temperature at the surface ; 
for among the most active volcanoes with which we are 
acquainted, are Hecla in Iceland, and Mount Erebus in the 
midst of the icy deserts which surround the south pole. 

199. It has been calculated that the temperature of 
the Earth increases as we descend at the rate of 1° Fahren- 
heit in a little over 50 feet. We shall therefore have a 
temperature of 

Fahr. Miles. 

212° or the temperature of ) , A . „ , t „ 

,.,. r , } at a depth of about 2 

boiling water . . ) r 

750° or the temperature of ) a « ^ 

red-hot iron . . . J 
1,850° or the temperature of ) it "is 

melted glass . . . j 
2,700° or the temperature at 

which everything we 

are acquainted with V 

would be in a state 

of fusion . . . . ^ 

200. If this be so, then the Earth's crust cannot exceed 
28 miles in thickness — that is to say, the i-Jo-th part of 

evidences have we of the interior heat ? 199. What is the rate of increase of 
temperature, as we descend helow the Earth's surface? At what depth would 
we have the temperature of boiling water ? The temperature of red-hot iron ? 
What temperature would we have at the depth of 18 miles ? At the depth of 28 
miles? 200. What follows, with respect to the thickness of the crust? What 



106 



THE EAETH. 



the radius ; so that it is comparable to the shell of an egg. 
But this question is one on which there is much difference 
of opinion, some philosophers holding that the liquid 
matter is not continuous to the centre, but becomes solid 
under the great pressure of the matter above. Indeed, 
evidence has recently been brought forward to show that 
the Earth may be a solid or nearly. solid globe from 
surface to centre. 

201. Density of the Earth's Crust. — The density of the 
Earth's crust is only about half of the mean density of the 
Earth taken as a whole. This has been accounted for by 
supposing that the materials of which it is composed are 
made denser at great depths than at the surface, by the 
enormous pressure of the overlying mass; but there are 
strong reasons for believing that the central portions are 
made up of much denser bodies than are common at the 
surface, — such as metals and the metallic compounds. 

202. The Flattening at the Poles explained. — It was 
prior to the solidification of its crust, and while the surface 
was in a soft or fluid condition, that the Earth put on its 
present flattened shape, the flattening being due to a 
bulging out at the equator, caused 

by the Earth's rotation. If we 
arrange a thin flexible hoop, as 
shown in Fig. 50, so that the 
upper part of it may move freely 
up and down on an axis, and then 
make it revolve 



very rapidly, 
it will assume 
an oval form, 
bulging out at 




Fig. 50. — Explanation of the Flattening at the 
Earth's Poles. 



those parts which are farthest from the axis, the motion 



opposite opinion is held "by some? 201. How does the density of the Earth's 
crust compare with that of the whole planet? How is this accounted for? 202. 
How is the flattening at the Earth's poles accounted for? Illustrate this with 



THE EARTH'S ATMOSFHERE. iq7 

being there most rapid, just as the Earth does at the 
equator. 

203. The form of the Earth, moreover, is exactly that 
which any fluid mass would take under the same circum- 
stances. This has been proved by placing a quantity of 
oil in a transparent liquid of exactly the same density as 
the oil. As long as the oil was at rest, it took the form of 
a perfect sphere floating in the middle of the fluid, exactly 
as the Earth floats in space; but the moment a slow 
rotary motion was given to the oil by means of a piece of 
wire forced through it, the spherical form was changed 
into a spheroidal one, like that of the Earth. 

204. Thus the tales told by geology, the still heated 
state of the interior, and the shape of the Earth, agree ; 
they all show that long ago the sphere was intensely 
heated and fluid. 

205. The Earth's Atmosphere. — We now pass to the 
atmosphere, which may be likened to a great ocean, cover- 
ing the Earth to a height not yet exactly determined. 
This height is generally supposed to be 45 or 50 miles, 
but there is evidence to show that we have an atmosphere 
of some kind at a height of 400 or 500 miles. 

206. The atmosphere is the home of the winds and 
clouds, and it is with these especially that we have to do, 
in order to understand the appearances presented by the 
atmospheres of other planets. Although in any one place 
there seems to be no order in the production of winds and 
clouds, on the Earth considered as a whole there is the 
greatest regularity. The Sun's heat and the Earth's rota- 
tion on its axis are, in the main, the causes of all atmos- 
pheric disturbances. 

Fig. 50. 203. What experiment, bearing on this flattening at the poles, has been 
made with oil? 204. What is the conclusion drawn respecting the condition of 
the Earth long ago ? 205. To what may the atmosphere ot the Earth be likened ? 
How high does it extend? 206. Of what is the atmosphere the home ? Is there 
any regularity in the production of winds and clouds ? What are the principal 
causes of atmospheric disturbances? 207. As regards calms and winds, into 



108 



THE EARTH. 



207. Belts of Calms and Winds. — If we examine a map 
showing the movements and conditions of the atmosphere, 
we shall find, encircling the Earth along the equator, a 
belt of Equatorial Calms. North of this we have the 
belt of Trade-winds, which blow from the north-east ; on 
jthe south we have a similar belt where the prevailing 
winds are south-east. 

Going from these belts toward the poles, we have on 
the north and south respectively the Calms of Cancer 
and the Calms of Capricorn. Still farther toward the 
Poles, we find the Anti-trades, blowing in the northern 
hemisphere from the south-west, and in the southern hemi- 
sphere from the north-west. At the poles there is a region 
of Polar Calms. 

208. Now, how are the winds just 
mentioned set in motion ? The equa- 
torial regions are the part of the 
Earth which is most heated; conse- 
quently the air there becomes rarefied 
and ascends, and a surface-wind sets 
in toward the equator on both sides 

\ I ^R\\W to fill its place : these are the trade- 
winds. The air thus wafted toward 
the equator is soon itself heated and 
ascends, and accumulating in the 
higher regions flows as an upper cur- 
Fig.51.-the trades and r ent toward either pole. Thus are 

Anti-trades. ., _ _ _ 

produced the anti-trades relerred to 
above, which in the regions beyond the calms of Cancer 
and Capricorn descend to the Earth's surface. The equa- 
torial belt (some 5° wide) in which the heated air is con- 
stantly ascending, is remarkable for daily rains, often 
accompanied with thunder and lightning. 




-* — ^. 



what successive belts do we find the Earth's surface divided ? 208. How are the 
trade-winds produced? How, the anti-trades ? For what is the equatorial belt 
remarkable? 209. What makes the trade- winds deviate in direction from due 



TRADE-WINDS AND ANTI-TRADES. 109 

209. If the Earth did not turn on its axis, we should 
still have the trade-winds, but they would blow due north 
and south from the poles to the equator. Their direction 
is modified by the Earth's rotation. Coming from higher 
latitudes with the less rapid rotary motion which there 
belongs to the Earth's surface, to the equatorial regions 
which have a more rapid motion, the Earth, as it were, 
slips from under them toward the east; and the winds, 
lagging behind, though really themselves also moving 
eastward, appear to come from the east, forming north- 
east winds north of the equator, and south-east winds 
south of it. 

In like manner, the anti-trades, endowed with the 
more rapid rotary motion of the equator, as they go 
toward the poles, arrive at regions where the rotary 
motion is less rapid. The Earth's surface, therefore, now 
lags behind, and the winds appear to blow, as they really 
do, toward the east, forming south-west winds in the 
northern hemisphere, and north-west winds in the southern. 

210. It is the Sun, therefore, that sets all this atmos- 
pheric machinery in motion, by heating the equatorial 
regions of the Earth ; and as the Sun changes its position 
with regard to the equator, crossing it twice in the course 
of the year, so do the calm-belt and trade-winds. The 
belt of equatorial calms follows the Sun northward from 
January to July, when it reaches 23-|° IN", lat., and then 
retreats, till at the next January it is in 23-J- S. lat. 

211. Clouds, Rain, Snow, Hail. — To the radiation from 
the Earth, combined with the fact that more or less 
watery vapor, or moisture, is always present in the air, 
must be ascribed the formation of mist and clouds, and 
the precipitation of rain, snow, and hail. The amount of 

north and south? Explain how the Earth's rotation operates on them. How 
does the Earth's rotation affect the anti-trades ? 210. How, and why, do the 
equatorial cahn-helt and the trade-winds change their position ? 211. To what 
are mist, cloud, rain, etc., due? Explain how clouds and rain are formed. TTn- 



HO THE EAETH. 

moisture that the air can hold varies with its temperature •, 
the warmer the air, the greater is its capacity for moisture. 
Hence, when air heavily charged with moisture derived 
by evaporation from the water-surfaces of the earth is 
chilled by a cold wind or contact with a mountain-side, 
its moisture is condensed into clouds or mist, and rain, 
snow, or hail, is formed. On the other hand, if a cloudy 
atmosphere is heated by the direct action of the Sun or 
by a current of warm air, its capacity for moisture is 
increased, and the clouds disappear. 

212. Chemical Elements of the Earth. — We now come 
to the materials of which the Earth, including its atmos- 
phere, is composed. These are 64 in number, and are 
called the Chemical Elements. They consist of 

Non-metallic [ Nitrogen, oxygen, hydrogen, chlorine, 

elements, bromine, iodine, fluorine, silicon, boron, 

or carbon, sulphur, selenium, tellurium, 

Metalloids. (^ phosphorus, arsenic. 

Metals of the alkalies : — Potassium, 
sodium, caesium, rubidium, lithium. 
Metallic J Metals of the alkaline earths : — Cal- 
elements. cium, strontium, barium. 

Other metals : — Aluminum, zinc, iron, 
tin, tungsten, lead, silver, gold, etc. 

Of these 64 elements, combined with each other for the 
most part in various ways, the Earth and every object that 
we see around us are composed. 

213. The elements which constitute the great mass of 
the Earth's crust are comparatively few — aluminum, cal- 
cium, carbon, chlorine, hydrogen, magnesium, oxygen, 
potassium, silicon, sodium, sulphur. Oxygen combines 



der what circumstances will clouds disappear ? 212. What is meant hy the Chem- 
ical Elements ? How many are there ? Into what two classes are they divided ? 
Name the non-metallic elements. Into what classes are the metallic elements 
subdivided? Name some of each class. 213. What elements constitute the 



COMPOSITION OF THE AIB. m 

with many of these elements, and especially with the 
earthy and alkaline metals ; indeed, about one-half of the 
Earth's crust is composed of oxygen in a state of com- 
bination. Thus sandstone, the most common sedimentary 
rock, is composed of silica, which is a compound of silicon 
and oxygen, and is half made up of the latter ; granite, a 
common igneous rock, composed of quartz, felspar, and 
mica, is nearly half made up of oxygen in a state of com- 
bination in those substances. 

214. Composition of the Air. — The chemical composi- 
tion, by weight, of 100 parts of the atmosphere at present 
is as follows : — Nitrogen, 11 parts ; Oxygen, 23 parts. 
Besides these two main constituents, we have 

Carbonic acid, . quantity variable with the locality. 
Aqueous vapor, . quantity variable with the tempera- 
ture and humidity. 
Ammonia, ... a trace. 

We said at present, because, when the Earth was 
molten, the atmosphere must have been very different. 
We had, let us imagine, close to the still glowing crust — 
consisting perhaps of acid silicates — a dense vapor, com- 
posed of compounds of the materials of the crust which 
were volatile only at a high temperature; the vapor of 
chloride of sodium, or common salt, would be present in 
large quantities ; above this, a zone of carbonic acid gas ; 
above this again a zone of aqueous vapor, in the form of 
steam ; and lastly, the nitrogen and oxygen. 

As the cooling went on, the lowest zone, composed of 
the vapor of salt and other chlorides, would be condensed 
on the crust, covering it with a layer of these substances 
in a solid state. Then it would be the turn of the steam 
to condense, and form water ; this would fall on the layer 

great mass of the Earth's crust? Of these, which enters most largely into the 
composition of matter ? Of what is sandstone composed ? Of what, granite ? 
214. What is at present the chemical composition of the air ? Give an account of 
the atmosphere, when the Earth was molten. As the cooling went on, what 



112 THE EAKTH. 

of salt, and dissolving it would form in time the ocean 
and seas, which would consequently be salt from the first 
moment of their appearance. Then, in addition to the 
nitrogen and oxygen which still remain, we should have 
the carbonic acid; this, in the course of long ages, was 
used up by its carbon going to form a luxurious vegeta- 
tion, the remains of which are still to be seen in the coal 
that warms us and does nearly all our work. 

215. It is the presence of vapor in our lower atmos- 
phere that renders life possible. When the surface of the 
Earth was hot enough to prevent the formation of the 
seas, as the water would be turned into steam again the 
instant it touched the surface, there could be no life. 
Again, if ever the surface of the Earth be cold enough to 
freeze all the water and all the gaseous vapor in the atmos- 
phere, life — as we have it — would be equally impossible. 

2 1 6. The Nebular Hypothesis, before alluded to, here 
comes in and teaches that, prior to the Earth's being in a 
fluid state, it existed as part of a vast nebula, the parent 
of the Solar System ; that this nebula gradually contracted 
and condensed, throwing off the planets one by one, some 
of which in turn threw off satellites ; and that its central 
portion, condensed perhaps to the fluid state, exists at 
present as the glorious heat-giving Sun. 

Although, therefore, we know that stars give out light 
because they are white-hot bodies, and that planets are 
not self-luminous because they are comparatively cold, we 
must not suppose that the planets were always cold, or 
that the stars will always be white-hot. There is good 
reason for supposing that all the planets were once white- 
hot, and gave out light as the Sun does now. 

changes took place ? What became of the carbonic acid ? 215. To what is the 
presence of vapor in the atmosphere essential ? Under what circumstances 
could there be no life ? 216. What does the nebular hypothesis teach respecting 
tbe former condition of the Earth ? What must we not suppose with regard to 
the planets and the stars ? What is there good reason to suppose respecting the 
planets ? 



SIZE AND DISTANCE OF THE MOON. 113 

CHAPTER VI. 
THE MOON. 

2 1 7. Size. — The Moon, as already stated, is one of the 
satellites, or secondary bodies ; and, although it appears to 
us at night to be infinitely larger than the fixed stars and 
planets, it is a little body but 2,153 miles in diameter. So 
small is it that 49 moons would be required to make one 
Earth, 1,245,000 earths being required, as we have seen, 
to make one Sun. 

218. Distance from the Earth. — The apparent size of 
the Moon, then, must be due to its nearness. This we find 
to be the case. The Moon revolves round the Earth in an 
elliptical orbit, having the Earth at one of its foci, at an 
average distance of only 237,640 miles, which is equal to 
about 10 times round our planet. As the Moon's orbit is 
elliptical, she is sometimes nearer to us than at others. 
The greatest and least distances are 253,263 and 221,436 
miles; the difference is 31,827 miles. When nearest us, 
of course she appears larger than at other times, and is 
said to be in perigee (nepi, near, and 777, the Earth) ; when 
most distant, she is said to be in apogee (<ztto, from, and 
777, the Earth). 

The Earth, by reason of its nearness, would of course look much 
larger to an observer on the Moon than any other of the heavenly bod- 
ies. When seen at the full, its disk would be as large as 13 full moons 
united would look to us. Bright spots would mark the continents, and 
the snow and ice about the poles ; dark spots would indicate the water- 
surfaces ; and variable ones, produced by the cloudy strata of the atmos- 
phere, would at times be distinguishable. 



217. To what class of heavenly bodies does the Moon belong? What is its 
size? What is its size, as compared with the Earth and Sun? 218. Why does 
the Moon look so large to us ? What is the shape of its orbit ? When is the 
Moon said to be in perigee f When, in apogee ? What is its mean distance from 
the Earth? What appearance would the Earth present to observers on the 



114 THE MOON. 

2 1 9. Period of Revolution. — The Moon travels round 
the Earth in a period of 27d. 7h. 43m. 11-Js. She requires 
more time to complete a revolution with respect to the 
Sun, which is called a Lunar Month, Lunation, or Synodic 
Period. 

220. Librations. — The Moon, like the planets and the 
Sun, rotates on an axis ; but there is this peculiarity in the 
case of the Moon, that her rotation and revolution round 
the Earth are performed in equal times. Hence we see only 
one side of our satellite. But, as the Moon's axis is in- 
clined 83° 21' to the plane of its orbit, we sometimes see 
the region round one pole, and sometimes the region round 
the other. This is termed Libration in latitude. 

There is also a Libration in longitude, arising from the 
fact that, though its rotation is uniform, its rate of motion 
round the Earth varies, so that we sometimes see more of 
the western edge, and sometimes more of the eastern. 
We have, moreover, a daily Libration, due to the Earth's 
rotation, carrying the observer to the right or left of a line 
joining the centres of the Earth and Moon. When on 
the right, or west, of this line, we should of course see 
more of the western edge of the Moon ; when to the left, 
in the case of an eastern position, we should see more of 
the eastern edge. 

By reason of these librations, instead of half the Moon's 
surface remaining constantly invisible, we see at one time 
or another about four-sevenths of its surface. 

221. Nodes. — The plane in which the Moon performs 
her journey round the Earth is inclined 5° to the plane of 

Moon, if there were any ? 219. What is the period of the Moon's revolution ? 
What is a Lunation? 220. How does the time of the Moon's rotation compare 
with that of her revolution ? What follows ? What is meant hy the Moon's Li- 
bration in Latitude ? What, by Libration in Longitude ? What other libration 
is there ? By reason of these librations, how much of the Moon's surface is at 
one time or another visible ? 221. What angle do the plane of the Moon's orbit 
and the plane of the ecliptic form ? What are the Nodes ? By what names are 
the nodes distinguished? 222. What renders the motion of the Moon compli- 
cated ? To what is its path round the Sun compared ? What is said of the devi- 



THE MOON'S ORBIT. 



115 



Fig. 



the ecliptic, in which the Earth performs her 
journey round the Sun (Art. 111). The two 
points in which the orbit of the Moon or 
any other celestial body intersects the Earth's 
orbit, are called the Nodes — that at which the 
body passes to the north of the ecliptic being 
distinguished as the Ascending Node, the other 
as the Descending Node. The line joining 
these two points is called the Line of Nodes. 

222. The Moon's Orbit. — The Moon revolves 
round the Earth in an ellipse which has the 
Earth at one of its foci ; but, while this revolu- 
tion is going on, the Earth also is moving, in an 
elliptical orbit which has the Sun at one of its 
foci. Hence (leaving out of view the fact that 
the Sun also has a motion in which it is ac- 
companied by both Earth and Moon) the mo- 
tion of the Moon is quite complicated. We 
may get an idea of its path round the Sun, if 
we imagine a wheel going along a road to have 
a pencil fixed to one of its spokes, so as to 
leave a trace on a wall: such a trace would 
consist of a series of curves with their concave 
sides downward, and such is the Moon's path 
with regard to the Sun. 

The total departure of the Moon from the 
Earth's orbit, however, does not exceed ^-g- of 
the radius of the Earth's orbit ; so that, unless 
drawn on a very large scale, the orbit of the 
Moon would appear to be identical with that 
of the Earth. In Fig. 52 the dotted line repre- 
sents the Earth's orbit; the continuous line, 
the Moon's. 

223. Earth-shine. — Besides the bright por- 



ation of the Moon's path from that of the Earth ? 223. Wheat is meant by Earth- 



116 THE MOON. 

tion lit up by the Sun, we sometimes see, in the phases 
which immediately precede and follow the New Moon, 
that the obscure part is faintly visible. This appearance 
is called the Earth-shine, and is due to that portion of 
the Moon reflecting to us the light it receives from the 
Earth. When this faint light is visible — when the " Old 
Moon " is seen " in the New Moon's arms " — the portion 
lit up by the Sun seems to belong to a larger moon than 
the other. This is an effect of what is called irradiation, 
and is explained by the fact that a bright object makes a 
stronger impression on the eye than a dim one, and ap- 
pears larger the brighter it is. 

224. The Moon's Light. — The average of four estimates 
gives the Moon's light as g47 1 gl3 of that of the Sun ; so we 
should want 547,513 full moons to give as much light as 
the Sun does. Now, there would not be room for so many 
in the half of the sky which is visible to us, as the full 
Moon covers g^oWo °f i* > hence it follows that the light 
from a sky full of moons would not be so bright as sun- 
shine. 

225. Apparent Difference of Size. — At rising or setting, 
the Moon sometimes appears to be larger than it does 
when high up in the sky. This is a delusion, and the re- 
verse of what we should expect ; for, as the Earth is a 
sphere, we are really nearer the Moon by half the Earth's 
diameter when the part of the Earth on which we stand 
is beneath it than we are at moonrise or moonset, when 
we are situated, as it were, on the side of the Earth, half- 
way between the two points nearest to and most distant 
from the Moon. Let the student draw a diagram, and 
reason this out. 

The larger appearance of the disk near the horizon is 
due simply to an error of judgment. It there seems placed 

shine, and how is it produced ? What difference of size is noticeable, and how 
is it explained ? 224. How does the Moon's light compare in brightness with 
the Sun's ? 225. When does the Moon appear largest? Why is this the reverse 



TELESCOPIC APPEAEANCE. nf 

beyond all the objects on the Earth's surface near the line 
along which we look, and therefore appears to be more 
distant than when it is overhead where there is no object 
near the line of view. Now, as it retains the same dimen- 
sions, and seems in the one case to be farther off than in 
the other, we intuitively endow it with a greater magni- 
tude in the situation which is apparently the most remote 
— and it appears to the eye accordingly. 

226. Telescopic Appearance. — A powerful telescope will 
magnify an object 1,000 times ; that is to say, it will enable 
us to see it as if it were a thousand times nearer than it is. 
If the Moon were 1,000 times nearer, it would be about 
240 miles off; consequently astronomers can see the Moon 
as if it were situated at this comparatively small distance, 
and they have studied and mapped with considerable ac- 
curacy the whole of the surface of our satellite which is 
turned toward us. 

227. "With the naked eye we see that some parts of the 
Moon are much brighter than others; there are dark 
patches, which, before large telescopes were in use, were 
thought to be oceans, gulfs, etc., and were so named. The 
telescope shows us that these dark markings are smooth 
plains, and that the bright ones are ranges of mountains 
and hill country broken up in the most tremendous man- 
ner by volcanoes of all sizes. A further study convinces 
us that the smooth plains are nothing but old sea-bottoms. 
In fact, once upon a time the Moon, like our Earth, was 
partly covered with water, and the land was broken up 
into hills and fertile valleys. 

As on the Earth we have volcanoes, so had the Moon, 
with the difference that the size, number, and activity of 
the lunar volcanoes were far beyond any thing we can 

of what we should expect? Explain why the Moon sometimes looks larger, 
when near the horizon. 226. How near does a powerful telescope bring the 
Moon? What have astronomers consequently heen ahle to do ? 227. What can 
we see with the naked eye, on the Moon's disk? What does the telescope show 
ihese dark patches and bright spots to be? What are the smooth plains found 



118 THE MOON. 

imagine. Several of the craters exceed 50 miles in diam- 
eter, and one of them even measures 114^ miles, — beside 
which that of Kilauea, in the Sandwich Islands, the largest 
terrestrial crater known (2-J- miles in diameter), dwarfs 
into insignificance. 

228. The best way of seeing how the surface of our 
satellite is broken up in this manner, is to observe the ter- 
minator — as the boundary between the bright and shaded 
portions is called. Along this line the mountain-peaks are 
lighted up, while the depressions are in shade ; and the 
shadows of the mountains are thrown the greatest distance 
on the illuminated portion. The heights of the mountains 
and depths of the craters have been measured by observ- 
ing the shadows in this manner. 

229. The Crater-mountains and Ranges of the Moon 
have been named after distinguished philosophers, astron- 
omers, and travellers. Thirty-nine peaks have been found 
whose height exceeds that of Mont Blanc (15,870 feet). 
Dorfel is 26,691 feet high; the Ramparts of Newton, 
measured from the floor of the crater, are 23,853 feet high; 
Eratosthenes is 15,750 feet. These heights, it must be re- 
membered, are much greater as compared with the size 
of the planet than the same elevations would be on the 
Earth's surface, as the Moon's diameter is but little more 
than one-fourth of the Earth's. 

230. The Crater Copernicus, one of the most prominent 
objects in the Moon, is represented below. The details 
of the crater itself and of its immediate neighborhood re- 
veal to us unmistakable evidences of volcanic action. The 
floor of the crater is strewn over with rugged masses, 
while outside the crater-wall (which on the left-hand side 
casts a shadow on the floor, as the drawing was taken 

to be ? What was once the case with respect to the Moon ? Describe the lunar 
craters. 228. What is the best way of seeing how the surface of the Moon is 
broken up ? 229. After whom have the crater ; moun tains and ranges of the Moon 
been named? How many peaks have been found higher than Mont Blanc? 
Mention some lunar peaks, and their height. 230. Describe the crater Ooperni- 



LUNAR CRATERS. 



119 



If" 




5 10 20 



Scale of f, ■■?„■■? ^ 



-? ^j Miles. 



The Lunar Crater Copernicus. 



soon after sunrise at Copernicus, and the Sun is to the left) 
many smaller craters are distinctly visible, those near the 
edge forming a regular line. Enormous unclosed cracks 
and chasms are also distinguishable. The depth of the 
crater-floor, from the top of the wall, is 11,300 feet; and 
the height of the wall above the general surface of the 
Moon is 2,650 feet. The irregularities in the top of the 
wall are well shown in the shadow. The scale of miles 



120 THE MOON. 

attached to the drawing shows the enormous proportions 
of the crater. 

231. Walled Plains, and curious markings called Rilles, 
are interesting features on the Moon's surface. The 
diameter of the walled plain Schickard, near the south- 
east limb or edge of the Moon, is 133 miles. Clavius and 
Grimaldi have diameters of 142 and 138 miles respectively. 

The rilles, of which 425 are now known, are trenches 
with raised sides more or less steep. Besides the rilles, at 
full Moon, bright rays are seen, which seem to start from 
the more prominent mountains. Some of these rays are 
visible under all illuminations. Those emanating from 
Tycho are different in their character from those emanating 
from Copernicus, while those from Proclus form a third 
class. 

Somewhat similar appearances have been produced on 
a glass globe by filling it with cold water, closing it up, 
and plunging it into warm water. This causes the en- 
closed cold water to expand very slowly, and the globe 
eventually bursts, its weakest point giving way, forming 
a centre of radiating cracks similar to the fissures, if they 
be fissures, in the Moon. 

232. Absence of Water and Atmosphere. — As far as we 
know (with possibly one exception, which is not yet 
established) the volcanoes of the Moon are now all ex- 
tinct ; the oceans have disappeared, and no water exists 
on its surface ; the valleys are no longer fertile ; nay, the 
very atmosphere has apparently left our satellite, and that 
little celestial body which probably was once the scene of 
various forms of life now no longer supports them. 

The absence of water and atmosphere may be accounted 
for by supposing that, on account of the small mass of the 

cus. 231. What other interesting features are there on the Moon's surface? 
What is the diameter of the walled plain Schickard ? Of Clavius ? Of Grimaldi ? 
What are the Rilles ? How many rilles are known ? What are seen apparently- 
starting from the more prominent mountains ? How have similar appearances 
been produced ? 332. What is the present condition of the Moon's surface? How 



ABSENCE OF ATMOSPHERE. 121 

Moon, its original heat has all been radiated into space. 
The cooling of its mass would be attended with contrac- 
tion, and the formation of vast caverns in the interior, 
which would communicate by fissures with the surface. 
In these internal receptacles the ocean may have been 
swallowed up, to such a depth that not even the fierce 
heat of the long lunar day can draw it forth in the form 
of vapor. The Moon, then, may be a picture of what the 
Earth is destined to be — revolving round the Sun, an arid 
and lifeless wilderness — if ever its internal heat be wholly 
lost. 

233. We say that the Moon has no atmosphere; (1) 
because we never see any clouds there, and (2) because, 
when the Moon gets between us and a star, the star dis- 
appears at once, and does not seem to linger on the edge, 
as it would do if there were an atmosphere. 

The lunar days must have a singular aspect. There 
being no atmosphere to diffuse the solar light, the fiery 
disk of the Sun stands out sharp and distinct against the 
background of the sky, everywhere else dark except 
where it is dotted with stars. There is no cloud, no wind, 
no twilight, no sound — but everywhere a silent and life- 
less desert. 

234. The Moon rotates on her axis, as we do, only 
more slowly ; hence the changes of day and night occur 
there as here. But instead of 24 hours, the Moon's day is 
29-J- of our days long ; so that each portion of the surface 
is in turn exposed to, and shielded from, the Sun for a 
fortnight. As there is no atmosphere either to shield the 
surface or to prevent radiation, it has been conjectured 
that the surface is, in turn, hotter than boiling water, and 
colder than any thing we have an idea of. 

may the absence of water be accounted for ? 233. Why do we say that the Moon 
has no atmosphere? What peculiarities must the lunar days present? 234. 
How long is the Moon's day ? With what change of temperature must the alter- 
nation of day and night be attended ? 235. What is meant by the Phases of the 
6 



122 



t:he moon. 



235. Phases of the Moon. — Let us now explain what 
are called the Phases of the Moon, — that is, the different 
shapes this luminary assumes. The Moon, like the Earth, 
gets all her light from the Sun. Now, it is clear that the 
Sun can only light up that half of the Moon which is turned 
toward it ; it is equally clear that, if we were on the same 
side of the Moon as the Sun is, we should see the lit-up 
half — if we were on the other side, being opposite the 
dark half, we should see nothing at all of the Moon. Now, 
this is exactly what happens. 

In Fig. 53 we suppose the plane of the Moon's orbit to 
correspond with the plane of the ecliptic, and the Sun to 




Fig. 53.— The Phases of the Moon. 

lie to the right ; the Moon's orbit is represented, and at 
its centre the Earth, the half turned toward the Sun being 
lighted up. When the Moon is at A, the illuminated side 
is away from the Earth, and we cannot see it ; this is the 

Moon ? In explaining these, what facts must first he taken into consideration ? 
With the aid of Fig. 53, explain the different phases. 23G. Mention in order the 



PHASES OF THE MOON. 123 

position occupied by the New Moon — and practically we 
do not see the New Moon at all. Now let us take the 
Moon at B ; at this point we face the lit-up portion and 
see all of it. Now, this occurs at Full Moon, when the 
Moon arrives at such a position in her orbit that the Sun, 
•the Earth, and herself, are in the same line, the Moon 
lying outside, and not between the other two as at the 
time of New Moon. 

At C and D our satellite is represented midway be- 
tween these two positions. At C one-half of the lit-up 
Moon is visible — that half lying to the right, as seen from 
the Earth ; at D we see the illuminated portion lying to 
the left, looking from the Earth. These positions are 
those occupied by the Moon at the First Quarter and 
Last Quarter respectively. When the Moon is at E and 
F, we see but a small part of the illuminated portion, and 
have a crescent Moon, the convex side in both cases being 
turned to the Sun. At G and H the Moon is said to be 
gibbous. 

236. We have, then, in succession, the following 
phases : — 
New Moon. The Moon is invisible to us, because the 
Sun is lighting up one side and we are on the 
other. 
Crescent Moon. We begin to see a little of the illumi- 
nated portion, but the Moon is still so nearly in 
a line with the Sun, that we only catch, as it 
were, a glimpse of the bright side, and that for a 
short time after sunset. 
First Quarter. As seen from the Moon, the Earth and 
Sun are at right angles to each other. When 
the Sun sets in the west, the Moon is south; 
hence the right-hand side of the Moon is illumi- 
nated. 

successive phases while the Moon is waxing. MeDtion the phases presented 



12 4 ECLIPSES. 

Gibbous Moon. The Moon is now more than half lighted 

up on the right-hand side. 
Full Moon. The Earth is now between the Sun and 
Moon, and therefore the entire bright half of the 
Moon is visible. 
From Full Moon we return, through similar phases in 
reversed order, to the New Moon, when the cycle recom- 
mences. So that, from New Moon the illuminated portion 
of our satellite waxes, or increases in size, till Full Moon, 
and then wanes, or diminishes, to the next New Moon; 
the illuminated portion, except at Full Moon, being 
separated from the dark one by a semi-ellipse, called, as 
we have seen (Art. 228), the Terminator. 



CHAPTER VII. 
ECLIPSES. 

237. In explaining the phases of the Moon, as repre- 
sented in Fig. 53, we supposed the motion of this luminary 
to be performed in the plane of the ecliptic ; but, as stated 
in Art. 221, this is not the case. If it were, every New 
Moon would put out the Sun ; and as the Earth, like 
every body through which light cannot pass, casts a 
shadow, every Full Moon would be hidden in that 
shadow. Such phenomena are called Eclipses, and they 
do happen sometimes; let us see under what circum- 
stances. 

238. Eclipses explained. — One-half of the Moon's jour- 
ney is performed above the plane of the ecliptic, one-half 
below it ; hence at certain times — twice in each revolution 

while the Moon is waning. How is the illuminated portion separated from the 
dark part ? 

237. If the Moon's orbit lay in the plane of the ecliptic, what would be the 



EXPLANATION OF ECLIPSES. 



125 




Fig. 54.— Explanation of Solab and Lunar 
Eclipses. 



— the Moon is in that 
plane, at those parts 
of it called the 
Nodes. Now, if the 
Moon, when in either 
node, happens to be 
in line with the Earth 
and Sun, we have an 
eclipse. If the Moon 
is new, it is directly 
between the Earth 
and the Sun, and the 
Sun is eclipsed. If 
the Moon is full, the 
Earth is between it 
and the Sun, and the 
Moon is eclipsed. 
This will be made 
clear by the accom- 
panying diagram. 

In Fig. 54 we 
have the Sun and 
the Earth, and the 
Moon in two posi- 
tions, A represent- 
ing it as new, and B 
as full. The level 
of the page repre- 
sents the plane of 
the ecliptic. We 
suppose, in both 
cases, that the Moon 
is at a node, — in the 
plane of the ecliptic, 
neither above nor 
below it. 



126 ECLIPSES. 

At A, the Moon stops the Sun's light ; its shadow falls 
on a part of the Earth, and the people, therefore, who live 
on that part cannot see the Sun, because the Moon is in 
the way. Hence we have what is called an eclipse of the 
Sun. 

At _Z?, the Moon is in the shadow of the Earth, and 
the Moon cannot receive any light from the Sun, because 
the Earth is in the way. Hence we have what is called 
an eclipse of the Moon. 

239. It will be seen from the figure, that whereas the 
eclipse of the Moon by the shadow of the much larger 
Earth will be more or less visible to the whole side of the 
Earth turned away from the Sun, the shadow cast by the 
small Moon in a solar eclipse is, on the contrary, so limited 
that the eclipse is seen over a small area only. 

240. Umbra and Penumbra. — In Fig. 54 two kinds of 
shadows are shown, one much darker than the other ; the 
former is called the Umbra (a Latin word, meaning shad- 
ow), the latter the Penumbra (from the Latin pcene, al- 
most, and umbra, a shadow). If the Sun were a point of 
light merely, the shadow would be all umbra ; but it is so 
large, that round the umbra, within which no part of the 
Sun is visible, there is a belt where a portion of it can be 
seen; hence we get a partial shadow, which constitutes 
the penumbra. This will be made clear if we take two 
candles to represent any two opposite edges of the Sun, 
place them rather near together, at equal distances from a 
wall, and observe the shadow they cast on the wall from 
any object ; on either side the dark shadow thrown by 
both candles, will be a lighter shadow thrown only by one. 

241. Total Eclipse of the Moon. — In a total eclipse of 

consequence ? 238. What is meant by the Nodes * Under what circumstances 
do eclipses take place ? Explain the occurrence of eclipses, with Fig. 54. 239. 
How do eclipses of the Moon and Sun compare, as regards the area over which 
they are respectively visible ? 240. What is the difference between the umbra 
and the penumbra ? How is the penumbra produced ? How may the formation 
of these shadows in an eclipse be illustrated? 241. Describe the several steps 



ECLIPSES OF THE MOON. 127 

the Moon, as this body travels from west to east, we first 
see its eastern side slightly dim as it enters the penumbra ; 
this is the first contact with the penumbra, spoken of in 
almanacs. At length, when the umbra is reached, the 
eastern edge becomes almost invisible, and we have the 
first contact with the dark shadow. The circular shape 
of the Earth's shadow is distinctly seen, and at last the 
Moon enters it entirely. Even then, however, the Moon's 
disk is scarcely ever wholly obscured ; the Sun's light is 
bent by the Earth's atmosphere toward the Moon, and 
sometimes tinges it with a ruddy color. 

A total eclipse of the Moon may last about If hours. 
When the Moon emerges from the umbra we have the 
last contact with the dark shadoio / then follows the last 
contact with the penumbra, and the eclipse is over. 

242. Partial Eclipse of the Moon. — If the Moon is very 
near, but not exactly at, a node, we have only a partial 
eclipse of the Moon, the degree of eclipse depending upon 
the distance from the node. For instance, if the Moon is 
north of the node, the lower limb may enter the upper 
edge of the penumbra or umbra ; if south of the node, the 
upper portion may be obscured. 

243. Total Eclipse of the Sun. — In a total eclipse of the 
Sun, the diameter of the shadow which falls on the Earth 
is never large, averaging about 150 miles. As the Moon, 
which throws the shadow, revolves from west to east in a 
month, while the Earth's surface, on which it falls, rotates 
from west to east in a day, the shadow travels more slowly 
than the surface, and so appears to sweep across it from 
east to west with great rapidity. The longest time an 
eclipse of the Sun can be total at any place is seven min- 
utes, and of course it is visible only at those places swept 



in a total eclipse of the Moon. How long may such an eclipse last ? 242. Under 
what circumstances have we a partial eclipse of the Moon? 2-13. What is the 
longest time an eclipse of the Sun can be total at any place ? Explain why it is 
so short. What follows Avith respect to the number of total eclipses of the Sun ? 



128 ECLIPSES. 

by the shadow. Hence, in any one place total eclipses of 
the Sun are very rare ; in London, for instance, prior to 
the total eclipse of 1 715, no such phenomenon had been 
visible for a period of 575 years. 

244. Annular Eclipse of the Sun.— When the Moon in- 
tervenes between the Sun and the Earth at such a distance 
from the latter as to make her apparent diameter less than 
the Sun's, a singular phenomenon is exhibited. The 
Whole disk of the Sun. is obscured except a narrow ring 
around the outside, encircling the darkened centre. This 
is called an Annular Eclipse (from the Latin a?mulus, a 
ring). Such an eclipse is represented in Fig. 55. 



Fig. 55.— Annular Eclipse of the Sun. 

To explain an Annular Eclipse fully, we must give a few figures. As 
both Sun and Moon are round, or nearly so, the shadow from the latter 
is round ; and as the Sun is larger than the Moon, the shadow ends in a 
point — its shape is, in fact, that of a cone, as shown in Fig. 55. Now, 
the length of this cone varies with the Moon's distance from the Sun, 
being of course shortest when the Moon is nearest to that luminary. The 

length of the Moon's shadow is about as follows : — 

Miles. 

When the Moon and Sun are nearest together, . . . 230,000 

" " " farthest apart, .... 238,000 

But the distance between the Earth and Moon varies as follows : — 

Miles. 
When the Moon and Earth are nearest together, . . . 225,719 
" " " farthest apart, .... 251,947 

Hence, when the Moon is farthest from the Earth, or in apogee, the shad- 
ow thrown by the Moon is not long enough to reach the Earth ; at such 
times the Moon looks smaller than the Sun, and, if she be at a node, we 
have an Annular Eclipse. 

245. Partial Eclipse of the Sun. — There may be Par- 

244. What is an Annular Eclipse of the Sun ? Under what circumstances is it 
produced? What is the shape of the Moon's shadow? Give figures to show 
that this shadow is not always long enough to reach the Earth. 245. Under 



EECUKREXCE OF ECLIPSES. 129 

tial Eclipses of the Sun, for the same reason that we have 
partial eclipses of the Moon. As the Moon is not exactly 
at the node, in the one case she is not totally eclipsed, be- 
cause she does not pass quite into the shadow of the 
Earth ; and in the other, the Sun is not totally eclipsed, 
because the Moon does not pass exactly between us and 
the Sun. 

246. To measure the extent of a partial eclipse, the di- 
ameter of the Sun or Moon, as the case may be, is divided 
into 12 equal parts, called digits, and the number of digits 
to which the greatest obscuration extends is stated. 

247. Recurrence of Eclipses. — The nodes of the Moon 
are not stationary, but move backward upon the Moon's 
orbit, a complete revolution taking place with regard to 
the Moon in 18 years 219 days, nearly. The Moon in her 
orbit, therefore, meets the same node again before she ar- 
rives at the same position with regard to the Sun, one 
period being 27 d. 5 h. 6 m., called the Nodical devolution 
of the Moon ; and the other, 29 d. 12 h. 44 m., called the 
Synodical devolution of the Moon. The node is in the 
same position with regard to the Sun after an interval of 
346 d. 14 h. 52 m. This is called a Synodic Revolution of 
the JSTode. 

iSTow, it so happens that nineteen synodical revolutions 
of the node, after which period the Sun and node would be 
alike situated, are equal to 223 synodical revolutions of the 
Moon, after which period the Sun and Moon would be 
alike situated. If, therefore, we have an eclipse at the 
beginning of the period, we shall have one at the end of 
it, the Sun, Moon, and node having returned to their orig- 
inal positions ; and all the eclipses of the period (with an 

■what circumstances is a partial eclipse of the Sun produced? 246. How is the 
extent of a partial eclipse measured ? 247. What motion have the nodes of the 
Moon ? What is meant by the Nodical Revolution of the Moon ? What is meant 
by its Synodical Revolution ? What is the length of each of these periods ? 
What is meant by the Synodic Revolution of a Node? How long a period is 
required for this revolution? Under what circumstances does a recurrence of 



130 ECLIPSES. 

occasional exception) will recur in the same order and at 
about the same intervals as before. This period of 18 
years 11 days 7 hours 40 min. 38 sec. (or, if 5 leap-years 
occur in the 18 years, 10 days instead of 11), was known 
to the ancient Chaldeans and Greeks under the name of 
/Saros, and by its means eclipses were predicted before as- 
tronomy had made much progress. 

248. Phenomena attending a Total Eclipse of the Sun.— 
A total eclipse of the Sun is at once one of the grandest 
and most awe-inspiring sights it is possible for man to wit- 
ness. As the eclipse advances, but before the disk is 
wholly obscured, the sky grows of a dusky livid, or purple, 
or yellowish crimson, color, which gradually gets darker 
and darker, and the color appears to run over large por- 
tions of the sky, irrespective of the clouds. The sea turns 
lurid red. This singular coloring and darkening of the 
landscape is quite unlike the approach of night, and gives 
rise to strange feelings of sadness. The Moon's shadow 
sweeps across the surface of the Earth, and is even seen in 
the air ; the rapidity of its motion and its intenseness pro- 
duce a feeling that something material is rushing over the 
Earth at a speed perfectly frightful. All sense of distance 
is lost ; the faces of men assume a livid hue, flowers close, 
fowls hasten to roost, cocks crow, birds flutter to the 
ground in fright, dogs whine, sheep collect together as if 
apprehending danger, horses and oxen lie down, obstinately 
resisting the whip and goad ; in a word, the whole animal 
world seems frightened out of its usual propriety. 

249. A few seconds before the commencement of the 
totality, the stars burst out, and surrounding the dark 
Moon on all sides is seen a glorious halo, generally of a 
silver- white light ; this is called the Corona. It is slightly 
radiated in structure, and extends sometimes beyond the 

eclipses take place ? After how long an interval ? What is this interval called f 
248. Describe a total eclipse of the Sun, as regards its effect on the sky, the 
landscape, and the animal world. 249. What is the Corona ? What are Aigrettes ? 



BAILY'S BEADS. 



131 



Moon to a distance equal to our satellite's diameter. Be- 
sides this, rays of light, called Aigrettes, diverge from the 
Moon's edge, and appear to be shining through the light 
of the corona. In some eclipses, parts of the corona have 
reached to a much greater distance from the Moon's edge 
than in others. It is supposed that the corona is the Sun's 
atmosphere, which is not seen when the sun itself is visi- 
ble, owing to the overpowering light of the latter. 

250. Sometimes 
when the advancing 
Moon has reduced 
the Sun's disk to a 
thin crescent, or in 
the case of an annu- 
lar eclipse to a nar- 
row ring, a peculiar 
notched appearance 
is presented in a 
part of the narrow 
strip, which makes 
it look like a string 
of beads (see Fig. 
56). This phenome- 
non has been called " Baily's Beads," from the astronomer 
Baily, who was the first to describe it. It is supposed to 
be the effect of irradiation. 

251. When the totality has commenced, close to the 
edge of the Moon, and therefore within the corona, are 
observed fantastically-shaped masses, bright red fading 
into rose-pink, variously called Red-flames and Red-promi- 
nences. Fig. 57 shows this phenomenon, as exhibited in 
the total eclipse of 1860. Two of the most remarkable of 
these prominences hitherto noticed, were observed in the 




-Annular Eclipse oe 
"Baily's Beads. 



Corona. 



What is the corona supposed to be ? 250. What is meant by "Baily's Beads"? 
What is supposed to produce them? 251. When the totality has commenced, 
what are sometimes observed within the corona? Describe these red-flames, as 



132 



ECLIPSES. 



eclipse of 1851 ; they were described by Mr. Dawes as fol- 
lows : — 

"A bluntly triangular 
pink body was seen suspend- 
ed, as it were, in the corona. 
This was separated from the 
Moon's edge when first seen, 
and the separation increased 
as the Moon advanced. It 
had the appearance of a 
large conical protuberance, 
whose base was hidden by 
some intervening soft and 
ill-defined substance. To the 
north of this appeared the 
most wonderful phenomenon 
of the whole : a red protu- 
berance, of vivid brightness 
and very deep tint, arose to 
a height of perhaps \y when 




Fig. 57.— Luminous Pkomtnences observed at 
Rivabellosa, Spain, during the total eclipse 
of the Sun, July 18th, 1860. 



first seen, and increased in length to 2' or more, as the Moon's progress 
revealed it more completely. In shape it somewhat resembled a Turkish 
cimeter, the northern edge being convex, and the southern concave. Tow- 
ard the apex it bent suddenly to the south, or upward, as seen in the 
telescope. To my great astonishment, this marvellous object continued 
visible for about five seconds, as nearly as I could judge, after the Sun 
began to reappear." 

252. It is certain that these prominences belong to the 
Sun, as those at first visible on the eastern side are grad- 
ually obscured by the Moon, while those on the western 
are becoming more visible, owing to the Moon's motion 
from west to east over the Sun. The height of some of 
them above the Sun's surface is upward of 40,000 miles. 
It is thought that they are incandescent clouds floating in 
the Sun's atmosphere, or resting upon the photosphere ; 
but this has not as yet been definitely established. 

253. Number of Eclipses. — In the Saros, or eclipse- 
observed by Mr. Dawes in the eclipse of 1851. 252. What reason is there for 
supposing that these prominences belong to the Sun ? What is the height of 
some of them? What are they thought to be? 253. How many eclipses occur 



NUMBEE OF ECLIPSES. 133 

period of 18 years 11 days, there usually happen 70 
eclipses, of which 41 are solar and 29 lunar. In any one 
year the greatest number that can occur is 7, and the least 
2 ; in the former case, 5 of them may be solar, and 2 lunar ; 
in the latter, both must be solar. Under no circumstances 
can there be more than 3 lunar eclipses in one year, and in 
some years there are none at all. Though eclipses of the 
Sun are more numerous than those of the Moon, in the 
proportion of 41 to 29, yet at any given place more lunar 
eclipses are visible than solar ; because, while the former 
are visible over an entire hemisphere, the latter are seen 
only in a narrow strip which cannot exceed 180, and i& 
usually about 150, miles in breadth. 

254. Memorable Eclipses. — Thales, of Miletus, one of 
the seven wise men of Greece, was the first to give the 
true explanation of eclipses. He predicted a total eclipse 
of the Sun, which took place 585 b. c, and is memorable 
for having put an end to an engagement between the 
Medes and Lydians. Herodotus tells us that the day was 
suddenly turned to night, and that, when the contending 
armies observed the strange phenomenon, they ceased 
fighting and concluded a peace which was cemented by a 
twofold marriage. 

Another total eclipse of the Sun, which occurred March 
1st, 557 b. c, led to the capture of the Median city Larissa 
by the Persians, its defenders having withdrawn from its 
walls in alarm. 

255. Effects of Eclipses on the "Uneducated. — Though 
the cause of eclipses was understood by the wise men of 
antiquity, the people generally, as indeed the uneducated 
in modern times have done until quite recently, regarded 

in the Saros ? What is the greatest, and what the least, numher that can occur 
in any one year? How many lunar eclipses may occur in a year? Which is 
oftener eclipsed, the Sun or the Moon ? Why are there more lunar than solar 
eclipses visible at any given place ? 254. Who was the first to give the true 
explanation of eclipses ? What eclipse did Thales predict ? What made this 
eclipse memorable ? What other memorable eclipse is mentioned ? 255. What 



134 ECLIPSES. 

these phenomena with dread. Savage nations, not unnat- 
urally, look upon them as omens of evil, and connect va- 
rious superstitions with their occurrence. 

The Hindoos, when they see the black disk of our sat- 
ellite advancing over the Sun, believe that the jaws of a 
dragon are gradually eating it up. To frighten off the 
devouring monster, they commence beating gongs and 
rending the air with discordant screams of terror and 
shouts of vengeance. For a time their efforts have no 
effect; the eclipse still progresses. At length, however, 
the uproar terrifies the voracious dragon ; he appears to 
pause, and, like a fish that has nearly swallowed a bait 
and then rejects it, he gradually disgorges the fiery 
mouthful. When the Sun is quite clear of the monster's 
jaws, a shout of joy is raised, and the exultant natives 
congratulate themselves on having, as they suppose, saved 
their deity from a disastrous fate. Elsewhere in India, 
the natives immerse themselves in the rivers up to the 
neck, which they regard as a most devout position, and 
thus seek to induce the luminary which is in process of 
eclipse to defend itself against the devouring dragon. 

An eclipse of the Moon, March 1st, 1504, proved of 
great service to Columbus. During one of his voyages 
of exploration, he was wrecked on the coast of Jamaica. 
Reduced to the verge of starvation by the want of provi- 
sions, which the natives refused to supply, he took advan- 
tage of their ignorance of astronomy to save himself and 
his men. Knowing that an eclipse of the Moon was about 
to take place, he called the natives around him on the 
morning before its occurrence, and informed them that the 
Great Spirit was displeased because they had not treated 
the Spaniards better, and would shroud his face from them 



has been the effect of eclipses on those unacquainted with their cause? What 
superstition do the Hindoos connect with a solar eclipse ? What position do the 
natives assume in some parts of India? How did Columbus once save himself 
and his men from great straits ? 



MERCURY. 135 

that night. When the Moon became dark, the Indians, 
convinced of the truth of his words, hastened to him with 
plentiful supplies, praying that he would beseech the 
Great Spirit to receive them again into favor. 



CHAPTER VIII. 
THE INFERIOR AND SUPERIOR PLANETS. 

256. To distinguish the planets which travel round the 
Sun within the Earth's orbit, from those which lie beyond 
it, the former, i. e., Mercury and Venus, are termed Infe- 
rior Planets: while the latter, i. e., Mars, Jupiter, Saturn, 
Uranus, and Neptune, are called Superior Planets. We 
proceed to consider these planets in turn. 

257. Mercury ($). — The nearest planet to the Sun 
whose existence is positively known, is Mercury. Under 
favorable circumstances, Mercury may be seen at certain 
times of the year for a few minutes after sunset, and then 
after an interval of some days for a few minutes before 
sunrise. At other times it keeps so close to the Sun as to 
be invisible, being lost in the superior brightness of his 
rays in the daytime, and setting and rising so nearly at 
the same time with him as to afford no opportunity of 
observation. It is never more than 29° distant from the 
Sun. 

To the naked eye Mercury looks like a star of the third 
magnitude, twinkling (unlike the other planets) with a 
pale rosy light. Viewed through the telescope, it exhibits 
similar phases to those of the Moon (from full to new) ; 

256. Into what two classes are the planets (the Earth excepted) divided ? 
Name the Inferior Planets. Name the Superior Planets. 257. Which is the 
nearest Planet to the Sun? At what times is Mercury visible ? Why is it not 
visible at other times ? What is its greatest distance in degrees from the Sun ? 



136 



THE INFEKIOK AND SUPEKIOK PLANETS. 



this is because we see more of its illuminated side at one 
time than another. Fig. 58 shows the phases of Mercury 
when seen after sunset. Its phases when seen before sun- 
rise are the same in reverse order, the illuminated part 
being turned in the opposite direction. 




Fig. 58.— Phases of Mercury, and its Comparative Size as seen at 
Different Times. 

258. Mercury's orbit is more elongated than that of 
any other of the principal planets ; it differs so much from 
a circle that at perihelion the planet is more than 14 mill- 
ions of miles nearer the Sun than at aphelion. It follows 
from this, and from the fact that Mercury is sometimes 
between us and the Sun and sometimes on the other side 
of the Sun, that the distance of Mercury from the Earth 
differs greatly at different times. The apparent diameter 
of Mercury as seen from the Earth varies accordingly, 
being at the planet's nearest point 2-J times as great as 
when it is farthest removed. This difference of size is 
represented in Fig. 58. 

259. The mean solar heat received at Mercury is nearly 
7 times as great as that of the Earth. The mean intensity 
of its light is also 7 times as great as ours, and the Sun 
seen from its surface would look 7 times as large as it 
does to us. We say its mean heat and light, for when it 



How does Mercury look to the naked eye ? What phases does it present, through 
the telescope? 258. Describe Mercury's orbit. How is it that Mercury's dis- 
tance from the Earth varies so much ? What change is noticeable in its appar- 
ent diameter ? 259. How do the heat and light received at Mercury compare with 



MERCURY. 137 

is nearest to the Sun the intensity of its heat and light is 
10 times as great as ours, and when most distant only 4-|- 
times as great. Hence the differences of temperature at 
different seasons are extreme ; the seasons, also, are of 
very unequal duration. Every six weeks on an average 
there is a change of temperature nearly equal to the dif- 
ference between frozen quicksilver and melted lead. 

260. Mercury turns on its axis in 24 hours 5-J minutes, 
and its year comprises about 88 of our days. The incli- 
nation of its equator to the plane of its orbit is believed 
to be considerably greater than the Earth's; if this be 
so, the relative length of the days and nights must vary 
more than on our planet. 

Mercury is the densest of the planets, having a specific 
gravity J greater than that of the Earth. The force of 
gravity on its surface is about f that on the Earth's sur- 
face. The flattening at the poles is much greater than 
that of our planet, the polar diameter being to the equa- 
torial as 28 to 29. 

261. The nearness of Mercury to the Sun prevents us 
from obtaining any very accurate knowledge of its sur- 
face. There are indications, however, of the existence of 
mountains, one of which, in the southern hemisphere, is 
estimated to be over 11 miles high. There are also evi- 
dences of an atmosphere. 

262. Venus ( ? ). — The second planet from the Sun is 
Venus. On account of its nearness, it appears larger and 
more beautiful to us than any other member of our plane- 
tary system. So bright is Venus that it is sometimes visi- 
ble by day to the naked eye, and at night in the absence 
of the Moon casts a perceptible shadow. Viewed through 

those of the Earth ? 2G0. How long are Mercury's day and year? What is be- 
lieved to he the case respecting the inclination of its axis ? What would fol- 
low ? How does Mercury compare with the Earth in density, and the force of 
gravity on its surface ? How does its polar diameter compare with its equato- 
rial ? 261. Of what are there indications on the surface of Mercury ? 2C2. Which 
is the second planet from the Sun ? Describe the appearance of Venus. What 



138 THE INFERIOR AND SUPERIOR PLANETS. 

the telescope, it presents phases similar to those of Mer- 
cury (see Fig. 58) and the Moon. When seen as a cres- 
cent, owing to its nearness to the Earth at that time, its 
apparent diameter is nearly 6^ times as great as when it is 
farthest off. 

263. Venus is always below the horizon at midnight. 
During part of the year, it rises before the Sun, and ushers 
in, as it were, the day ; when appearing at this time, the 
ancients styled it Phosphor or Lucifer (the light-bearer), 
and we call it the Morning Star. A few days after it 
ceases to be visible in the morning, it appears after sun- 
set ; it was then styled Hesperus or Vesper by the an- 
cients, and is distinguished by us as the Evening Star. 
The greatest distance it attains from the Sun is 47°. 

264. In size, density, and the force of gravity on its 
surface, Venus differs but little from the Earth. No flat- 
tening at the poles has been observed, from which it is 
inferred that in this respect also it resembles the Earth, as 
the flattening on our planet is so small that it would be 
imperceptible to an observer on Venus. In consequence 
of the nearly circular form of its orbit, its four seasons are 
nearly uniform in length ; but the great inclination of its 
equator to the plane of its orbit (49° 58') must, as in the 
case of Mercury, make a great difference in the relative 
length of day and night, and subject its polar regions to 
extreme changes of temperature. Venus'sdayis about 23^ 
hours long, and its year is equal to about 224| of our days. 

265. Venus's heat and light are twice as intense as 
ours. A dense, cloudy atmosphere is believed to envelop 
the planet. Spots have been observed on its surface ; and 
irregularities are seen in the terminator, which are sup- 
change does its apparent diameter undergo, and why? 263. When is-Venus dis- 
tinguished as the Morning, and when as the Evening, Star? What is its greatest 
distance from the Sun ? 264. In what respects does Venus differ little from the 
Earth ? In what other respects does it resemhle the Earth ? What must follow 
from the great inclination of its axis to the plane of its orbit ? How long are 
Venus's day and year? 265. How do the heat and light of Venus compare with 
the Earth's ? What is believed to envelop the planet ? What do the irregulari- 



MAES. 



139 



posed to indicate lofty mountains, in some cases exceeding 

20 miles in height. 

266. Mars ( $ ). — Mars, fourth in order from the Sun, is 

the nearest to 
the Earth of the 
superior planets. 
Its day is of 
nearly the same 
length as ours ; 
its year is about 
twice as long 
as our year ; 
its diameter is 
two-thirds that 
of the Earth. 
The inclination 
of its axis to the 
plane of its or- 
bit does not dif- 
fer much from 
the Earth's, and 
its seasons are 
therefore similar 
to ours. When 
it is nearest to 
the Earth, its 
apparent diam- 
eter is seven 
times as large 
as when it is 
farthest off. 

267. In Fig. 
59 are presented 




Fig. 59.— Mars in 1862. 



ties iu llie terminator indicate ? 266. Which of the superior planets is nearest 
to the Earth ? How does Mars compare with the Earth, as regards its day, its 
year, its diameter, and its seasons ? How and why does its apparent diameter 



140 THE INFEKIOR AND SUPEKIOK PLANETS. 

two sketches of Mars, taken in 1862. Here we have some- 
thing strangely like the Earth. The shaded portions rep- 
resent water, the lighter ones land, and the bright spot at 
the top of the drawings is probably snow lying round the 
south pole. 

The two drawings represent the planet as seen in a tele- 
scope, which inverts objects, so that the south pole of the 
planet is shown at the top. In the upper drawing, which 
was made on the 25th of September, a sea is seen on the left, 
stretching down northward; while, joined to it, as the 
Mediterranean is to the Atlantic, is a long, narrow sea, 
which widens at its termination. In the lower drawing, 
made September 23 d, this narrow sea is represented on 
the left. The coast-line on the right reminds one of the 
Scandinavian peninsula, and the included Baltic Sea. 

268. It will now be seen how we are able to determine 
the length of a planet's day and the inclination of its axis. 
We have only to watch how long it takes one of the spots 
near the equator to pass from one side to the other, and 
the direction in which it moves, to get at both these facts. 

269. For a reason that will be understood when we 
come to deal with the effect of the Earth's revolution round 
the Sun on the apparent positions and aspects of the 
planets, we sometimes see the north pole of Mars, some- 
times its south pole, and sometimes both. When but one 
pole is visible, the features which appear to pass across 
the planet's disk in about twelve hours — that is, half 
the period of the planet's rotation — describe curves with 
the concave side toward the visible pole. When both 
poles are visible they describe straight lines, exactly as in 
the case of the Sun (Art. 110). These changes enable all 
the surface to be seen at different times, and maps of Mars 



vary ? 267. Describe the surface of the planet, as represented in two sketches 
taken in 1862. 268. How can we determine the length of a planet's clay and the 
inclination of its axis ? 269. What is the direction of the features which cross 
the disk of Mars? How is the exact position of these features, as laid down in 



MAES. 141 

have been constructed, the exact position of the features 
of the planet .being determined by their latitude and lon- 
gitude, as in the case of the Earth. 

270. Mars has not only land, water, and snow, like the 
Earth, but also clouds and mists, and these have been 
watched at different times. The land is generally reddish 
when the planet's atmosphere is clear ; this is due to the 
absorption of the atmosphere, as is the color of the setting 
Sun with us. Hence the fiery red light by which Mars is 
distinguished in the heavens. The water appears of a 
greenish tinge. 

271. Now, if we are right in supposing that the bright 
spot surrounding the pole is ice and snow, we ought to 
see it rapidly decrease in the planet's summer. This is 
actually found to be the case, and the rate at which the 
thaw takes place is one of the most interesting facts to be 
gathered from a close study of the planet. In 1862, this 
decrease was very visible. The summer solstice of Mars 
occurred on the 30th of August, and the snow-zone was 
observed to be smallest on the 11th of October, or forty- 
two of our days after the highest position of the Sun. This 
very rapid melting may be ascribed to the inclination of 
the planet's axis, the great eccentricity of its orbit, and 
the fact that the summer of the southern hemisphere occurs 
when the planet is near perihelion. 

272. Though we see in Mars so many things that re- 
mind us of the Earth, and show us that the extreme tem- 
peratures of the two planets are not far from equal, in one 
respect they differ widely. In consequence of the great 
eccentricity of the orbit of Mars, the seasons are not so 
nearly equal in length as with us, and owing to the longer 

maps, determined ? 270. How is Mars distinguished in the heavens ? To what 
is this red light attributahle ? What tinge has the water on the surface of Mars ? 
271. What is the bright spot observed near the south pole of Mars supposed to 
be ? What interesting fact was observed in connection with this spot in 1862 ? 
To what may the rapid thaw be ascribed ? 272. In what respect does Mars dif- 
fer widely from the Earth ? What is the length of the several seasons in the 



142 



THE INFEKIOK AND SUPEEIOE PLANETS. 



year, they are of much greater extent. In the northern 
hemisphere of the planet, 

days. hrs. days. hrs. 

Spring lasts 191 8 | Autumn lasts 149 8 

Summer " 181 I Winter " 147 

As we must reverse the seasons for the southern hemi- 
sphere, spring and summer, taken together, are 76 days 
longer in the northern hemisphere than in the southern. 

273. Jupiter 01 ). — Passing over the asteroids, which 
will be considered by themselves in the next chapter, we 
come to Jupiter, by far the largest planet of our system. 
Jupiter exceeds the Earth in bulk nearly 1,400 times. 
Its revolution round the Sun is performed in about 12 of 
our years, with a velocity 80 times as great as that of a 




Fig. 60.— Comparative Size of Jupiter and the Earth.— Belts of Jupiter. 

cannon-ball. It turns on its axis in less than 10 hours. 
The flattening at its poles is still greater than that at Mer- 



northern hemisphere of the planet? How do the spring and summer of the 
southern hemisphere compare in length with those of the northern ? 273. Which 
is the largest planet of the Solar System ? How does it compare in hulk with 
the Earth ? What are its periods of revolution and rotation ? How do we find 



JUPITEE. 143 

cury's, its polar diameter being to its equatorial diameter 
as 16 to 17. This flattening, represented in Fig. 60, is 
what we should expect from its very rapid rotation (Art. 
202). 

274. Jupiter stands nearly upright in its orbit, the incli- 
nation of its axis being only about 3°. Hence in any given 
part of the surface there is very little change of season. 
In the equatorial regions, summer reigns throughout the 
year ; the temperate zones rejoice in perpetual spring ; 
while around the poles winter continually prevails. The 
polar day is 6 years long, and is followed by a night of 
equal length. Owing to the deviation of its orbit from a 
circle, the planet is 46 millions of miles nearer the Sun at 
perihelion than at aphelion, and receives in consequence -} 
more heat. 

275. Jupiter appears to the naked eye like a star of 
the first magnitude, bright enough sometimes, when the 
Moon is absent, in spite of its great distance, to cast a 
shadow like Venus. A glance at its disk, as represented 
in Fig. 60, shows us that we have something very unlike 
Mars. In fact it is surrounded by an atmosphere so densely 
laden with clouds, that of the actual surface we know, 
nothing. 

276. What are generally known as the belts of Jupiter 
are dusky streaks which cross a brighter background in 
directions generally parallel to the planet's equator. They 
are sometimes seen in large numbers, and extend almost 
to the poles. The largest belts are, for the most part, 
situated on either side of the equator, just as the two belts 
of trade-winds on the Earth lie on either side of the belt 
of equatorial calms. Outside these, again, we have the 
calms of Cancer and Capricorn represented, although these 

its shape to have been affected by its rapid rotation ? 274. What is the inclina- 
tion of Jupiter's axis ? What follows, respecting the seasons ? How much more 
heat does the planet receive at perihelion than at aphelion ? 275. How does Ju- 
piter appear to the naked eye ? By what is Jupiter surrounded ? 276. Describe 
the belts of Jupiter, and their relative positions. What tint has the equatorial 



144 THE INFERIOR AND SUPERIOR PLANETS. 

belts are not so regularly seen, the portion of the planet's 
surface in which they are sometimes visible being liable 
to great changes of appearance, in a comparatively short 
time. The portions of the atmosphere representing the 
terrestrial calm-belts sometimes exhibit a beautiful rosy 
tint, the equatorial one especially. 

Besides the belts, spots are seen, sometimes bright and 
sometimes dark, which have enabled us to determine the 
period of the planet's rotation. Its rotary velocity is so 
great that on the equator an observer would be carried 
round at the rate of 467 miles a minute, instead of 17 as 
on the Earth. This rapid rotation would necessarily break 
the cloudy surface into belts more than in our case or that 
of Mars. In the latter planet, indeed, no trace of cloud- 
belts has as yet been detected ; their absence is perhaps 
due to its slow rotation and small size. 

277. Though all astronomers do not agree that the 
surface of the planet is never seen, there are many good 
reasons for supposing this to be the case. In the first 
place, Mars and the Earth, whose atmospheres are nearly 
alike, have nearly the same density (Art. 157) ; while the 
density of Jupiter (and the same reasoning applies to Sat- 
urn, whose belts, as far as we can observe them, resemble 
Jupiter's), if vihat we see is all planet, is only about one- 
fifth that of the Earth, or not far from that of water. 
Now, there is no reason to suppose that the matter of 
which Jupiter is composed dingers so very widely in char- 
acter from that of Mars and the Earth. It seems more 
probable that the apparent volume of Jupiter (and, in like 
manner, that of Saturn) is made up of a large shell of 
cloudy atmosphere and a kernel of planet, and that the 
density of the real Jupiter (and the real Saturn) may not 
differ very much from that of the Earth. 

belt? What are seen "besides the belts ? What is the planet's rotary velocity ? 
What is the necessary effect of this rapid rotation on the atmosphere ? To what 
is the absence of cloud-belts in Mars due ? 277. What reason is there for sup- 



THE MOONS OF JUPITEK. 



145 



Moreover, our own planet was most probably envel- 
oped in a great shell of cloudy atmosphere, in one of the 
early stages of its history, before its crust had cooled down 
(Art. 214). The future ocean of Jupiter may now in like 
manner be spread about him, in the form of a blanket of 
cloud, 20,000 miles or more in thickness. 

278. Besides the changing features of Jupiter itself, the 
telescope reveals to us four moons, which, as they course 
along rapidly in orbits lying nearly in the plane of the 
planet's orbit, lend additional interest to the picture. In 




Fig. 61.— Jupitek and his Four Moons. 

their various positions in their orbits, the satellites some- 
times appear at a great distance from their primary ; some- 
times they come between us and the planet, appearing now 
as bright and now as dark spots on its surface. At other 
times they pass between the planet and the Sun, throwing 
their shadows on the planet's disk, and causing, in fact, 
eclipses of the Sun. They also enter the shadow cast by 
the planet, and are therefore eclipsed themselves; and 
sometimes they pass behind the planet, and are said to be 
occulted. Of these appearances we shall have more to say 
hereafter. 

The passage of either a satellite or shadow is called a 
Transit. In a solar eclipse, could we observe it from Ve- 



posing that Jupiter and Saturn are enveloped in immense shells of cloudy atmos- 
phere ? In what was oar own planet probably once enveloped ? 278. How many 
moons has Jupiter? In what different positions does the telescope exhibit 
them? What is meant by a Transit? 279. How do Jupiter's moons compare 



146 



THE INFEEIOE AND SUPEEIOK PLANETS. 



nus, we should see the shadow of our Moon sweeping over 
the Earth's surface. 

279. Referring to the sizes of these satellites and their 
distances from the planet, in Table III. of the Appendix, 
we find that all but one are larger than our Moon, and that 
all are farther from their primary than our Moon is from 
us. Like our Moon, they rotate on their axes in the same 
time as they revolve round their primary. This is inferred 
from the fact that their light varies, and that they are al- 
ways brightest and dullest in the same positions with re- 
gard to Jupiter and the Sun. 

280. Saturn ("£>). — Saturn, which is next to Jupiter in 
distance from the Sun, is also next to it in size, having a 
volume about 750 times that of the Earth. Its day is 
not half so long as ours, but it is 29-J of our years in mak- 
ing one complete revolution in its orbit. 

281. Saturn, which 
is belted like Jupiter, 
is surrounded not only 
by eight moons, but 
by a series of rings, 
the innermost one of 
which is transparent. 
The belts have been 
already referred to 
(Art. 277). Seven of 
the moons were known for sixty years before the eighth 
was discovered. Their diameters, distances from their 
primary, etc., are given in Table III. of the Appendix. 
The equator of Saturn, unlike that of Jupiter, is greatly 
inclined to the ecliptic ; transits, eclipses, and occupations 




Satukn and its Moons. 



with our Moon in size? How, in distance from the primary? In what respect 
do they resemble our Moon ? From what is this inferred ? 280. What planet 
ranks next to Jupiter in size ? How does Saturn compare with the Earth in 
size? How do Saturn's day and year compare with ours? 281. How many 
moons has Saturn ? By what else is it surrounded ? In what respect is Saturn 
vei"y unlike Jupiter? In what plane do the orbits of Saturn's satellites mostly 



THE BINGS OF SATUEN. 147 

of the satellites, the orbits of which for the most part lie 
in the plane of the planet's equator and rings, happen but 
rarely. 

282. It is to the rings that most of the interest of this 
planet attaches. We may imagine how sorely puzzled the 
earlier observers, with their very imperfect telescopes, 
were, by these strange appendages. The planet at first 
was supposed to resemble a vase ; hence the name Answ, 
or handles, given to the rings in certain positions of the 
planet. It was next supposed to consist of three bodies, 
the largest in the middle. The true nature of the rings 
was discovered by Huyghens in 1655, who announced it 
in this curious form : — 
"aaaaaaa ccccc d eeeee g h iiiiiii 1111 mm 

nnnnnnnnn 0000 pp q rr s ttttt uuuuu." 
These letters, placed in their proper order, read : — "Annu- 
lo cingitur tenui piano, nusquam cohcerente, ad eclipticam 
inclinato." — "It is surrounded by a thin flat ring, nowhere 
attached to its surface, inclined to the ecliptic." 

There is nothing more encouraging in the history of 
astronomy than the way in which eye and mind have 
bridged over the tremendous gap that separates us from 
this planet. By degrees the fact that the appearance was 
due to a ring was determined; then a separation was no- 
ticed, dividing the ring into two; the extreme thinness 
of the ring came out next, when Sir William Herschel 
observed the satellites "like pearls strung on a silver 
thread ; " then an American astronomer, Bond, discovered 
that the number of rings must be multiplied, we know not 
how many fold. The transparent ring was next made out 
by Dawes and Bond, in 1852; then the transparent ring 

lie ? What is said of the occurrence of transits, eclipses, and occnltations of the 
satellites? 282. What constitute the most interesting feature of this planet? 
What was Saturn at first supposed to resemble? What were the rings first 
called? Who discovered the true nature of the rings, and when? How did 
Huyghens announce his discovery ? State the discoveries successively made re- 



148 THE INFEEIOE AND SUPEEIOE PLANETS. 

was discovered to be divided as the whole system had 
once been thought to be ; last of all comes evidence that 
the smaller divisions in the various rings are subject to 
change, and that the ring-system itself is probably increas- 
ing in breadth, and approaching the planet. 

283. Fig. 63 will give an idea of the appearance pre- 
sented by Saturn and its strange but beautiful appendage ; 
also of its size as compared with the Earth. It will be 
shown in Chapter XII. that we see sometimes one surface 
of the ring-system, sometimes another, and occasionally 
only its edge. 




Fig. 63.— Saturn and the Eakxh— Comparative Size. 

284. The ring-system is situated in the plane of the 
planet's equator, and its dimensions are as follows :— 



specting the rings of Saturn. 283. Do wc always see the same part of the ring- 
system ? 284. In what plane is the ring-system situated ? What is the distance 



THE KINGS OF SATUEN. 



149 



Outside diameter of outer ring, . 

Inside " 
Distance from outer to inner ring, 
Outside diameter of inner ring, . 
Inside " " 

Inside " dark ring, . 

Distance from dark ring to planet 
Equatorial diameter of planet, 

So that the breadths of the three principal 
the entire system, are as follows : — 



Miles. 

166,920 
147,670 



91,780 

9,760 

71,900 

rings, and of 

Miles. 



Outer bright ring, 9,625 

Inner bright ring, 17,605 

Dark ring, 8,660 

Entire system, 37,570 

In spite of this enormous breadth, the thickness of the 
rings is not supposed to exceed 100 miles. 

285. Of what, then, are these rings composed ? There 
is great reason for believing that they are neither solid nor 
liquid. The idea now generally accepted is that they are 
composed of myriads of little satellites, moving indepen- 
dently, each in its own orbit, round the planet ; giving rise 
to the appearance of a bright ring when they are closely 
packed together, and a \erj dim one when they are most 
scattered. In this way we may account for the varying 
brightness of the different parts, and for the haziness on 
both sides of the ring near the planet (shown in Fig. 84), 
which is supposed to be due to some of the satellites being 
drawn out of the ring by the attraction of the planet. 

286. Although Saturn appears to resemble Jupiter in 
its atmospheric conditions, its year, unlike that planet's, 

from the planet to the nearest dark ring? What is the breadth of the three prin- 
cipal rings respectively? What is the hreadth of the entire system? What is 
the thickness of the rings ? 285. Of what are the rings now generally believed to 
consist? What are accounted for on this supposition? 286. What follows from 
the great inclination of Saturn's axis, with respect to its seasons ? By what be- 



150 THE INFEEIOE AND SUPEEIOE PLANETS. 

and like our Earth's, owing to the great inclination of its 
axis, is sharply divided into seasons. Saturn's seasons, 
however, are marked by something else than a change of 
temperature ; we refer to the effects produced by the pres- 
ence of its ring-appendage. To understand these effects, 
its appearance from the body of the planet must first be 
considered. As the plane of the ring-system lies in the 
plane of the planet's equator, an observer at the equator 
will only see its edge, and the rings will therefore look 
like a band of light passing through the east and west 
points and the zenith. As the observer, however, increases 
his latitude either north or south, the surface of the ring- 
system will begin to be seen, and will gradually increase 
in width. As it widens, it will also recede from the zenith, 
until in lat. 63° it is lost below the horizon ; and between 
this latitude and the poles it is altogether invisible. 

Now, the plane of the ring always remains parallel to 
itself, and twice in Saturn's year — that is, in two opposite 
points of the planet's orbit — it passes through the Sun. It 
follows, therefore, that during one-half of the revolution of 
the planet one surface of the rings is lit up, and during 
the remaining period the other surface. At night, in 
the one case, the ring-system will be seen as an illumi- 
nated arch, with the shadow of the planet passing over it, 
like the hour-hand over a dial ; and in the other, if it be 
not lit up by the light reflected from the planet, its posi- 
tion will be indicated only by the entire absence of stars. 

287. But if the rings eclipse the stars at night, they 
can also eclipse the Sun by day. In latitude 40° we have 
morning and evening eclipses for more than a year, grad- 
ually extending until the Sun is eclipsed during the whole 

sides a change of temperature are its seasons marked ? How would the rings be 
presented to an observer at the equator of Saturn ? As he increases his latitude, 
what changes will be exhibited in the rings ? When will the rings sink below the 
horizon? State the facts with respect to the illumination of the surfaces of the 
rings. How must the illuminated surface look at night ? How is the dark sur- 
face indicated? 287. What phenomena are produced by the rings in the day- 



UK ANUS. 151 

day — that is, when its apparent path lies entirely in the 
region covered by the ring. These total eclipses continue 
for nearly 7 years, and eclipses of one kind or another take 
place for 8 years 292 days. This will give us an idea how 
largely the apparent phenomena of the heavens, and the 
actual conditions as to climates and seasons, are influenced 
by the presence of the ring. 

As the year of Saturn equals 2 9 J- of our years, it fol- 
lows that each surface of the rings is in turn deprived of 
the light of the Sun for nearly 15 years. 

288. Uranus (W)- — Uranus, the next planet to Saturn, 
usually shines as a star of the sixth magnitude, and is just 
visible to the naked eye. It is 72 times larger than the 
Earth, .and revolves about the Sun in 84 of our years. 
There being no spots on its surface, we are unable to fix 
the period of its revolution on its axis. The intensity of 
the solar heat and light received at Uranus are only 3^- 
of ours. 

Uranus is attended by four moons, which are remark- 
able for their retrograde motion, travelling in the oppo- 
site direction to that of all the other planets, except the 
moon of Neptune, and in orbits nearly perpendicular to 
the orbit of their primary. Owing to the great distance 
of Uranus, nothing is known of its physical peculiarities, 
except that its specific gravity is about -§- that of the 
Earth — a little greater than the specific gravity of ice. 

289. Neptune (¥). — Neptune, the most remote planet 
of the solar system and the third in size, is invisible to the 
naked eye. Seen through the telescope, it looks like a 
star of the eighth magnitude. Its revolution round the 
Sun is performed in about 165 of our years. It has one 

time ? Describe the eclipses in lat. 40°. How long is each surface of the rings 
\n turn deprived.of the Sun's light? 288. What appearance does Uranus pre- 
sent? How do its size, its year, and the intensity of its light and heat compare 
with ours ? How many moons has Uranus ? For what are they remarkable ? 
What is the specific gravity of Uranus ? 289. Which is the most remote planet 
of our system? How does Neptune look? How long is its year? How many 



152 THE ASTEROIDS, OR MINOR PLANETS. 

moon, a little nearer to its primary than our Moon is to 
us. Its light and heat have but -j-oVo the intensity of ours. 
Nothing is known of the period of rotation or the atmos- 
pheric conditions of Neptune. Its density is about ^ of 
the Earth's, or not quite equal to that of sea-water. 



CHAPTER IX. 



290. Bode's Law. — If we write down 

3 6 12 24 48 96 

and add 4 to each, we get 

4 V 10 16 28 52 100 
and this series of numbers represents very nearly the dis> 
tances of the ancient planets from the Sun, as follows : — 

Mercury, Yenus, Earth, Mars, — , Jupiter, Saturn. 
This singular fact was discovered by Titius, and is known 
by the name of Bode's Law. We see that the fifth term 
has apparently no representative among the planets. This* 
fact acted so strongly on the imagination of Kepler that 
he boldly placed an undiscovered planet in the gap. 

291. Discovery of the Asteroids. — Up to the time of the 
discovery of Uranus, the suspected planet had not revealed 
itself; when it was found, however, that the position of 
Uranus was very well represented by the next term of 
Bode's series, 196, it was determined to make an organized 
search for it. For this purpose a society of astronomers 
was formed ; the zodiac was divided into 24 zones, and 
each zone was confided to a member of the society. On 

moons has it? How do Neptune's light, heat, and density, compare with those 
of the Earth? 

290. What is meant hy "Bode's Law" ? Which term of the series was not 
represented among the planets ? 291. What gave an impetus to the search for 



THEIE SIZE AND QEBITS. 153 

the first day of the present century a planet was discovered 
and named Ceres, which, curiously enough, filled up the 
gap. The discovery of a second, third, and fourth, named 
respectively Pallas, Juno, and Vesta, soon followed; and 
up to the present time (Feb., 1873) no less than 126 of 
these little bodies have been detected. See Table I., 
Appendix. 

292. Size of the Asteroids. — None of these planets, 
except occasionally Ceres and Vesta, can be seen by the 
naked eye. This is owing to their small size ; the largest 
minor planet is but 228 miles in diameter, and many of 
the smaller ones are less than 50. The force of gravity 
on their surfaces must be very small. A man placed on 
one of them would spring with ease 60 feet high, and 
sustain no greater shock in his descent than he does on 
the Earth from leaping a yard. On such planets giants 
may exist; and those enormous animals which here re- 
quire the buoyant power of water to counteract their 
weight, may there inhabit the land. 

293. Orbits. — The orbits of the asteroids thus far dis- 
covered, for the most part, lie nearer to Mars than Jupiter, 
and are in some cases so elliptical, that, if we take the ex- 
treme distances into account, they occupy a zone 240,000,000 
miles in width — the distance between Mars and Jupiter 
being 336,000,000. The planet nearest the Sun is Flora, 
whose journey round that luminary is performed in 3^ 
years, at a mean distance of 201,000,000 miles. The most 
distant one is Maximiliana, whose year is as long as 6^ 
of ours, and whose mean distance is 313,000,000 miles. 

There is a resemblance between the orbits of some of 
the asteroids and those of comets, not only in their degree 

the undiscovered planet? What plan was adopted? What was the result? 
What further discoveries have since heen made ? 292. Which of the asteroids are 
occasionally visihle to the naked eye ? What is the size of the asteroids ? What 
follows, with respect to the force of gravity? 293. How do the orbits of the aste- 
roids thus far discovered lie ? How wide a zone do they occupy? Which of the 
asteroids is nearest the Sun ? Which is the most distant ? How do their mean 
distances and years compare ? In what respects do the orbits of some of the as- 



154 



THE ASTEROIDS, OR MINOR PLANETS. 



of eccentricity, but also in their great inclination to the 
plane of the ecliptic. The orbit of Pallas, for instance, is 
inclined to this plane at an angle of 34° ; that of Mas- 
silia, on the other hand, nearly coincides with it. 

294. Evidences of Atmosphere and Rotation. — Pallas 
has been supposed, fr@m its hazy appearance, to be sur- 
rounded by a dense atmosphere, and this may also be the 
case with the others, as their colors are not the same. 
There are also evidences that some among them rotate on 
their axes, like the larger planets. 

295. Mode of Discovery, — The minor planets lately dis- 
covered shine as stars of the tenth or eleventh magnitude ; 
and the only way in which they can be detected, there- 
fore, is to compare the star-charts of different parts of the 
heavens with the heavens themselves, night after night. 
Should any point of light not marked on the chart be ob- 
served, it is immediately watched, and if any motion is 
detected, its rate and direction are determined. In the 

Fig. 64. 




latter case, either a new planet or a comet has been dis- 
covered. 



teroids resemble those of the comets ? How do the orbits of Pallas and Massilia 
differ in inclination to the plane of the ecliptic ? 294. Of what are there evidences 
in the case of some of the asteroids ? 295. Describe the mode of discovering the 



THEOKY AS TO THEIR ORIGIN. 155 

Fig. 64, which represents on the left the star-map and on the right 
the field of view of the telescope, will give an idea of the method pur- 
sued. The stars shown in both are the same, and in the field of the 
telescope is seen the new body, which, absent from the map, and changing 
its position relatively to the stars, is found to be a member of our solar 
system. 

296. Theory respecting the Asteroids. — To account for 
the existence of the asteroids, it has been suggested that 
they may be fragments of a larger planet destroyed by 
contact with some other celestial body. One fact seems, 
above all, to indicate an intimate relation between ail the 
minor planets : it is, that if their orbits are figured under 
the form of material rings, these rings will be found so 
entangled that it would be possible, by means of one 
among them, taken at hazard, to lift up all the rest. 

It is probable that the largest of the asteroids have 
been discovered; yet as more powerful instruments are 
used, many new ones, less easily visible from the Earth, 
will no doubt from time to time be added to their number. 
According to Le V«errier's computation, the total mass of 
these bodies revolving between the orbits of Mars and 
Jupiter is such that, allowing them to equal the Earth in 
density and to have an average size equal to that of such 
as have been already discovered, their whole number 
would be not far from 150,000 ! 



CHAPTER X. 

COMETS. 

297. We have seen that round the white-hot Sun cold 
or cooling solid bodies, called planets, revolve; that be- 

minor planets. 296. What theory has been advanced, to account for the exist- 
ence of the asteroids ? What fact indicates an intimate relation between them ? 
What is probable with regard to future discoveries ? How great may the num- 
ber of the asteroids be ? 



156 



COMETS. 



cause they are cold they do not shine by their own light ; 
that they perform their journeys in almost the same plane ; 
that the shape of their orbits is oval or elliptical; and 
that they all move in one direction, — that is, from west to 
east. 

But these are not the only bodies which revolve round 
the Sun. There are, besides, masses, probably white-hot, 
called Comets (from the Greek tiou,r\rr\q, long-haired)^ 
which shine by their own light ; which perform their 
journeys round the Sun in every plane, in orbits some of 
which are so elongated that they can scarcely be called 
elliptical ; and — a further point of difference — while some 




Fig. 65.— Cometaet Orbits. 



297. What have we learned with respect to the planets ? What other bodies 
have we now to consider? How do comets differ from planets? 298. What 



THE COMETAEY OKBITS. 



157 



move round the Sun in the same direction as the planets, 
others revolve from east to west. 

298. Orbits of the Comets, — The orbits of the comets 
are either ellipses, parabolas, or hyperbolas (see Fig. 65). 
Comets that move in elliptical orbits revolve round the 
Sun in definite periods. Their paths are exceedingly 
elongated, the cometary orbit which most nearly ap- 
proaches a circle (that of Faye's comet) having a much 
greater eccentricity than the planetary orbit which departs 
most from a circle (the asteroid Polyhymnia's). 

Comets that describe parabolas or hyperbolas will 
never return, as these curves consist of two constantly- 
diverging branches. After once visiting our system, they 
go away forever, seeking perhaps another sun in the 
depths of the heavens. Some of those, however, that 
appear to describe parabolas may really move in greatly- 
elongated ellipses and return after very long periods. 

Here is a list of some comets whose period is known : — 





Time of 


Nearest Ap- 


Greatest Dis- 


Next 
Return. 


COMETS. 


Revolu- 
tion. 


proach to the 
Sun. 


tance from the 
Sun. 




Tears. 








Encke's . . . 


H 


32,000,000 


387,000,000 


1872 


De Yico's . . 


«i 


110,000,000 


475,000,000 


1872 


Winnecke's 


5i 








1875 


Brorsen's . . 


&h 


64,000,000 


537,000,000 


1873 


Biela's . . . 


6ir 


82,000,000 


585,000,000 


1873 


D'Arrest's . . 


&h 








1870 


Faye's . . . 


% 


192,000,000 


603,000,000 


1873 


Mechain's . . 


™l 








1871 


Halley's . . 


76| 


56,000,000 


3,200,000,000 


1910 



curves do the comets describe ? What comets return at fixed periods ? How do 
the elliptical cometary orbits compare in shape with the planetary orbits ? What 
comets will never return ? Mention some of the short-period comets, and their 
time of revolution. Mention some of the long-period comets, and their time of 



158 



COMETS. 



These are called Short-period Comets. Of the Long- 
period Comets we may mention those of 1858, 1811, and 
1844, whose periods of revolution have been estimated at 
2,100, 3,000, and 100,000 years respectively. 

299. Distances from the Sun. — From the table given 
above it will be seen how the distance of these erratic 
bodies from the Sun varies at different points of their or- 
bits. Thus Encke's comet is twelve times nearer the Sun 
at perihelion than at aphelion. Some comets whose aphelia 
lie far beyond the orbit of Neptune, at perihelion almost 
graze the Sun's surface. Sir Isaac Newton estimated that 
the comet of 1680 approached so near the Sun that its 
temperature was two thousand 
times that of red-hot iron ; at 
its nearest point, it was but 
one-sixth part of the Sun's di- 
ameter from the surface. The 
comet of 1843 also made a very 
near approach to the Sun, and 
was visible in broad daylight. 

300. Appearance presented 
by a Comet. — In Fig. 66 we 
give a representation of Do- 
nati's Comet, visible in 1858, 
which will serve to illustrate 
a general description of these 
bodies. The brighter part of 
the comet is called the head, 



or coma / and sometimes the 




Don ati's Comet (general 
view). 



head contains a brighter por- 
tion still, called the nucleus. The tail is the dimmer part 
flowing from the head ; as observed in different comets, it 



revolution. 299. Give instances showing how the distance of some comets from 
the Sun varies at different points of their orhits. What facts are stated respect- 
ing the comets of 1680 and 1843 ? 300. Name and describe the different parts of a 
comet. What different appearances does the tail present? What evidence is 



CHANGES IN THEIR APPEARANCE. 159 

may be long or short, straight or curved, single, double, or 
multiple. The comet of 1*744 had six tails, that of 1823 
two. In some comets, particularly those whose period of 
revolution is shortest, the tail is entirely wanting. 

Both head and tail are so transparent that all but the 
faintest stars are easily seen through them. In 1858, the 
bright star Arcturus was visible through the tail of Do- 
nates comet, at a place where the tail was 90,000 miles in 
diameter. 

301. Changes in Appearance. — When these bodies are 
far away from the Sun, their heat is feeble, and their light 
dim ; we observe them in our telescopes as round misty 
bodies, moving very slowly, say a few yards in a second, 
in the depths of space. Gradually, as the comet ap- 
proaches the Sun, and its motion increases (for the nearer 
any body, be it planet or comet, gets to the Sun, the faster 
it travels), the Sun's action begins to be felt, the comet 
gets hotter, gives out more light, and becomes visible to 
the naked eye. 

A violent action soon commences ; the gas bursts forth 
in jets from the coma toward the Sun, and is instantly 
driven back again, as the steam of a locomotive going at 
great speed is driven back on its path, though from a dif- 
ferent cause. The jets rapidly change their position and 
direction, and a tail is formed, which seems to consist of 
the smoke or products of combustion driven off from the 
coma, probably by the repulsive power of the Sun, and 
rendered visible by his light. The tail is always turned 
away from the Sun, whether the comet be approaching or 
receding from that body. 

As the comet gets still nearer to the Sun, and there- 
fore to the Earth, we begin to see in some instances that 
the coma contains a nucleus, brighter than itself; the jets 

there that both head, and tail are transparent ? Mention a case in point. 301. 
What changes of appearance take place, as a comet approaches the Sun? In 
what direction is the tail always turned ? What does the coma sometimes con- 



160 



COMETS. 




are distinctly visible, and some- 
times the coma consists of a 
series of envelopes. This was 
the case in the beautiful comet 
of 1858; the nucleus was con- 
tinually throwing off these en- 
velopes, which surrounded it 
like the layers of an onion, and 
peeled off, expanding outward 
and giving place to others. 
Seven distinct envelopes were 
thus seen ; as they were driven 
off, they seemed to be expelled 
into the tail. 

Hence the tails of comets, 
as a rule, rapidly increase as 
they approach the Sun, which 
gives rise to all this violent ac- 
tion. The tail of the comet of 1861 was 20,000,000 miles 
long, and this length has been exceeded in many cases. The 
tail of the comet of 1843 was 112,000,000 miles long, the 
diameter of the coma being 112,000 miles, that of the nu- 
cleus 400 miles ; near perihelion, the tail increased at the 
rate of 35,000,000 miles a day. 

Halley's comet, as observed by Sir John Herschel, and 
Encke's comet, are exceptions to the above rule. As these 
bodies approached the Sun, both tail and coma decreased, 
and the whole comet appeared only like a star. Still, most 
comets increase in brilliancy, and their tails lengthen, as 
they near the Sun, — so much so that in some instances 
they have been visible in broad daylight. The enormous 
effect of a near approach to the Sun may be gathered from 
the fact that the comet of 1680, at its perihelion passage, 



Fig. 67.— Donati's Comet (show- 
ing the Head and Envelopes). 



tain ? What does the nucleus in some cases throw off? What does the tail gen- 
erally do, as the comet gets still nearer the Sun ? What comets were exceptions 
to this rule? Give the length of the tails of two or three comets. 332. What 



DANGER FROM COLLISIONS. 161 

while it was travelling at the rate of 1,200,000 miles an 
hour, in two days shot out a tail 60,000,000 miles long. 

302. Danger from Collisions. — In old - times, when less 
was known about comets, they caused great alarm ; not 
merely superstitious terror, which connected their coming 
with the downfall of a king or the outbreak of a plague, 
but a real fear that they would dash our planet to pieces 
should they come into contact with it. Modern science 
teaches us that in the great majority of instances the mass 
of the comet is so small that we need not be alarmed ; in- 
deed, there is good reason to believe that on June 30th, 
1861, we actually passed through the tail of the glorious 
comet which then suddenly appeared, the only noticeable 
phenomenon being a peculiar phosphorescent mist. Again 
in 1776, a comet approached so close to Jupiter as to be- 
come entangled among its satellites, but the latter all the 
time pursued their way as if the comet had never existed. 
'This, however, was by no means the case with the comet ; 
it was thrown entirely out of its course, its orbit was 
changed, and it has ceased to be a long-period comet, 
its revolution being now accomplished in about twenty 
years. 

303. A Divided Comet. — There is an instance on record 
of a comet's dividing into two portions, which afterward 
pursued distinct but similar orbits. This is Biela's comet, 
given in the table in Art. 298. But this is not all. These 
twin comets were due again at perihelion at the end of 
January, 1866, and ought to have been visible from the 
Earth on the 30th of November ; but in spite of the strict- 
est watching nothing was seen of them. It is believed that, 
like Lexell's comet, they have been diverted from their 
course by some member of our system, and that in this 



apprehension was formerly felt respecting comets ? What does modern science 
teach us ? What two occurrences show us that there is little cause for alarm ? 
303. What remarkahle facts are mentioned in connection with Biela's comet? 
What is thought to have heen the disturbing cause ? 304. What have we reason 



162 COMETS. 

case the November meteors may have been the disturbing 
cause. 

304. Physical Constitution of Comets. — In the case of a 
comet without a nucleus, we have reason to believe that 
the coma is a mass of white-hot gas, like that of which the 
nebulae are composed; whether a comet with a nucleus 
is made up of similar matter we do not know. One thing 
is certain, that as the tail indicates the waste, so to speak, 
of the head, each return to the Sun must reduce the mass 
of the comet. 

A diminution of velocity would in time reduce the 
most refractory comet into a quiet member of the solar 
family, as the orbit would become less elliptical, or more 
circular, at each return to perihelion. This effect has, in 
fact, been observed in some of the short-period comets. 
Encke's comet, for instance, now performs its revolution 
round the Sun in three days less than it did eighty years 
ago. This reduction of speed has been attributed to the 
resistance offered by the ethereal medium — a resistance not 
noticed in the case of the planets, because their mass is 
so much greater. 

Sir Isaac Newton calculated that a cubic inch of air at 
the Earth's surface — that is, as much as is contained in a 
good-sized pill-box — if reduced to the density of the air 
4,000 miles above the surface, would be sufficient to fill a 
sphere the circumference of which would be as large as 
the orbit of Neptune. The tail of the largest comet, if it 
be gas, may therefore weigh but a few ounces or pounds ; 
and the same argument may be applied to the comet itself, 
if it be not solid. With so limited a supply there is not 
room for waste, and in the case of so small a mass the re- 
sistance offered may easily become noticeable. 



to believe that a comet consists of? What is certain with respect to each return 
to the Sim? What has been observed in connection with Encke's comet? To 
what is this shortening of the period of revolution attributed ? What calculation 
was made by Newton ? How much, then, may the tail of the largest comet 



HOW FOEMEELY EEGAEDED. 163 

305. Number of Comets. — From the earliest times, be- 
ginning with the Chinese annals, to our own, about 800 
comets have been recorded ; but the number observed at 
present is much greater than formerly, as many are re- 
vealed by the telescope. It is thought that there may be 
many millions of these bodies belonging to our system, 
and perhaps passing between it and other systems. We 
see but few of them, because those only are visible to us 
which are favorably situated for observation when they 
pass the Earth in their journey to or from perihelion; 
while there may be thousands which at their nearest ap- 
proach to the Sun are beyond the orbit of Neptune. 

306. Comets, how formerly regarded. — Comets were in 
ancient times regarded as the harbingers of war, pestilence, 
and famine. " A fearful star, for the most part, this comet 
is," says Pliny, " and not easily appeased." The Sertorian 
War, the civil dissensions between Pompey and Caesar, 
and the cruelties of Nero, were all, as the Romans thought, 
foreshadowed by these celestial messengers of ill. A 
comet of terrific sword-like appearance, according to Jose- 
phus, seemed to hang over Jerusalem, a. d. 69, inspiring 
the inhabitants with the greatest alarm ; which was real- 
ized by the destruction of the city the following year. 

As late as the year 1456, all Europe was thrown into 
consternation by the appearance of a comet, which in the 
minds of the terrified people presaged victory to the Turks, 
who were at that time pressing hard upon Christendom. 
We are told that the bells were rung at noon every day, 
and prayers offered for preservation " from the Devil, the 
Turk, and the Comet." 

weigh, if it be gas ? 305. How many comets have been recorded ? How does 
the number now observed compare with that formerly known ? Why is this? 
How many comets, is it thought, may belong to our system ? Why do we see so 
few of them? 306. How were comets regarded in ancient times? What does 
Pliny say of them? What events in Roman history were thought to have been 
foreshadowed by comets ? By what was the destruct ion of Jerusalem preceded ? 
What took place as late as the year 1453 ? What did the Norman Chronicle argue 
from the comet of 1066 a. d. ? 



164 



COMETS. 



The Norman Chronicle refers to a comet which ap- 
peared in the year of the Norman Conquest, 1066, as evi- 
dence of the divine right of William of Normandy to in- 
vade England. 



CHAPTER XI 



METEORS AND METEORITES. 

307. Number of Meteors. — There are very few clear 
nights in the year in which, if we watch for some time, we 
shall not see one of those appearances which are called, 
according to their brilliancy, Meteors, Bolides, or Shooting- 
Stars. On some nights we may even see a shower of fall- 
ing stars, and the shower in certain years is so dense that 
in some places the number seen at once equals the appar- 
ent number of the fixed stars seen at a glance. It has 
been calculated that the average number of meteors which 

traverse the atmosphere 
daily, and which are large 
enough to be visible to the 
naked eye on a dark clear 
night, is no less that 7,500,- 
000 ; and, if we include mete- 
ors which would be visible in 
a telescope, this number will 
be increased to 400,000,000 ! 

Some astronomers have even ac- 
counted for the Zodiacal Light, seen 
at certain seasons, in the east before 
sunrise, and in the west after sun- 
z set, by attributing it to an immense 

collection of these minute bodies, 




Fig 



-Shape of the 

LIGHT. 



307. What are visible on almost any clear night ? On some nights what may 
we even see ? How many meteors, on an average, has it been calculated, traverse 
the atmosphere daily ? To what have some astronomers attributed the Zodiacal 



. THE NOVEMBER SHOWERS. 165 

which the Earth is thus constantly encountering, and which on entering 
our atmosphere become meteors. Too small to be seen separately, even 
with the telescope, they combine in countless multitudes, according to 
this hypothesis, to reflect to our eyes the light in question, borrowing it 
from the Sun round which they revolve. The shape of the Zodiacal 
Light is that of a cone, with its base turned toward the Sun, and its 
axis nearly in the plane of the ecliptic, as shown in Fig. 68. 

308. Theory of the November Showers. — It is now gen- 
erally held that these little bodies are not scattered uni- 
formly in the space comprised by the Solar System, but are 
collected into several groups, some of which travel, like com- 
ets, in elliptic orbits round the Sun ; and that what we call a 
shower of meteors is due to the Earth's breaking through 
one of these groups. Two such groups are well denned, 
and we break through them in August and November in 
certain years. The exquisitely beautiful star-shower of 
1866 has placed the truth of this explanation beyond all 
doubt. 

To explain this theory further, we must again fall back 
upon our imaginary ocean (Art. Ill) to represent the plane 
of the ecliptic. Let us also suppose that the Earth's path 
is marked out by buoys placed at every degree of longi- 
tude, beginning from the place occupied by the Earth at 
the autumnal equinox. Now, if it were possible to buoy 
space in this way, we should see the November group of 
meteors rising from the plane at the point occupied by our 
Earth about the 14th of November. 

309. But why do we not have a star-shower every No- 
vember ? Because the meteors are not uniformly distrib- 
uted throughout their orbit, but are mostly collected in a 
great group in one part of it. To have a dense shower, we 
must not only cross their orbit, but cross it at a time when 
the principal group of little bodies is in that part of it 

Light? What is the shape of this light? 308. What opinion is now generally 
held respecting meteoric showers ? When do we break through two well-defined 
groups ? Illustrate this theory as regards the November group, taking an imagi- 
nary ocean to represent the plane of the ecliptic. 399. Why do we not have 



166 METEOES AND METEOEITES. 

which we are crossing. Now, the group referred to per- 
forms its revolution in about 33^ years ; hence No- 
vember showers may be expected at intervals of about 33 
years. But as the meteors extend along their orbit in a. 
great stream so far that it takes two or three years for them 
to clear the node, or point of their path at which they 
cross the Earth's orbit, a shower, more or less brilliant, 
proceeding from different parts of the same lengthened 
group, may occur in two or three consecutive years. 

310. About a dozen of these November showers are re- 
corded. One of them occurred November 12th, 1799; 
another, of remarkable brilliancy, on the 13th of Novem- 
ber, 1833, when it was estimated that 575 fell on an aver- 
age each minute. On the 14th of November, 1866, and 
also on the same day of the two following years, there was 
a recurrence of these remarkable phenomena. 

311. The Radiant-point. — Now, what will happen when 
the Earth, sailing along in its path, reaches the node and 
encounters the mass of meteoric dust, the particles of which 
are travelling in the opposite direction ? 

Let us in imagination connect the Earth and Sun 
with a straight line. At any moment, the direction of the 
Earth's motion will be at right angles to this line (or, as 
it is called, a tangent to its- orbit) ; therefore, as longitudes 
are reckoned from right to left, the motion will be directed 
to a point 90° of longitude behind the Sun. The Sun's 
longitude at noon on the 14th of November, 1866, was 
232°, within a few minutes ; 90° from this gives us 142°. 

Since therefore the meteors, as we encounter them in 
our journey, should seem to come from the point of space 
toward which the Earth is travelling, and not from either 
side, we ought to see them coming from a point situated 

star-showers every November ? How long an interval occurs between succes- 
sive showers ? 310. How many November showers are recorded ? Mention three 
that have taken place. 311. In what direction do the meteoric particles travel, as 
regards the Earth ? If our theory is correct, from what longitude ought the me- 
teors to appear to come ? Explain why this is so. Now, what was actually no- 



THE KADIANT-POINT. 167 

in longitude 142°, or thereabout. Now, what was actually- 
seen ? 

One of the most striking facts, noticed even by those 
who did not see its significance, was that all the meteors 
seen in the star-shower referred to seemed to come from 
one part of the sky. In fact, there was a region in which 
the meteors appeared trainless, and shone out for a mo- 
ment like so many stars, because they were directly ap- 
proaching us. Near this spot they were so numerous, and 
all so foreshortened and for the most part faint, that the sky 
at times put on almost a phosphorescent appearance. As 
the eye travelled from this region, the trains increased in 
length, those being longest as a rule which first made their 
appearance overhead, or which trended westward. Now, 
ii* the paths of all had been projected backward, they 
would have intersected in one region, that, namely, in 
which the most foreshortened ones were seen. In fact, 
there were moments in which the meteors belted the sky 
like the meridians on a terrestrial globe, the pole of the 
globe being represented by a point in the constellation Leo. 
From this point they all seemed to radiate, and Radiant- 
point is the appropriate name given to it by astronomers. 
Now, the longitude of this point is 142°, or thereabout. 

The apparent radiation from this point is an effect of 
perspective ; hence we gather that the paths of the meteors 
are parallel, or nearly so, and that the meteors themselves 
all travel in straight lines from the radiant-point. 

312. Orbits of the Meteors. — By careful observations 
of the radiant-point it has been determined that the orbit 
of each member of the November star-shower, and there- 
fore of the whole mass, is an ellipse with its perihelion 
lying on the Earth's orbit, and its aphelion just beyond 
the orbit of Uranus ; that its inclination to the plane of 

ticed ? What is meant by the Radiant-point ? How is it situated ? To what is 
the apparent radiation from this point due ? What conclusions do we draw 
from this ? 312. What has been established with respect to the orbit of the me- 



168 METEORS AND METEORITES. 

the ecliptic is 1 7° ; and that the direction of the motion 
of the meteors is retrograde. 

Up to the present time 56 such radiant-points have 
been determined, which possibly indicate 56 similar 
groups moving round the Sun in cometary or planetary 
orbits. The meteors of particular showers vary in their 
distinctive characters, some being larger, brighter, and 
more ruddy, than others — some swifter, and drawing after 
them more persistent trains, than those of other showers. 

3 i 3. Cause of the Luminous Appearance. — Let us now 
take the case of a single meteor entering our atmosphere ; 
why does it present such a brilliant appearance ? 

In the first place, we have the Earth travelling at the 
rate of 1,100 miles a minute, plunging into a mass of bodies 
whose velocity, at first equal to its own, is soon increased 
to 1,800 miles a minute by the Earth's attraction. The 
meteoric body enters our atmosphere at this rate — 30 
miles a second. Its motion is soon arrested by the friction 
of that atmosphere, which puts a brake on it, as it were, — 
and it becomes hot, just as the wheel of a tender gets hot 
under similar circumstances, or a cannon-ball when the 
target impedes its further flight. So hot does it get that, 
at last, as great heat is always accompanied by light, we 
see it ; it becomes vaporized, and leaves a train of lumi- 
nous vapor behind it. 

Heat results from the stoppage of any mechanical force, in proportion 
to the amount of force stopped. The number of meteors is known to be 
immense ; multitudes beyond conception may exist in the planetary 
spaces ; it has been thought, as already stated, that the Zodiacal Light 
may even be due to a great ring of these little bodies surrounding the 
Sun and reflecting its light. Now, putting these facts together, some 
philosophers have attempted to account for the. constant supply of solar 
heat, kept up without perceptible diminution from century to century, 



teors of the November star-shower ? How many radian t-points have been deter- 
mined np to the present time ? What, possibly, do these indicate ? In what 
respects do the meteors of particular showers vary ? 313. What is the cause of 
the luminous appearance of meteors ? How have some explained the constant 



AUGUST AND APEIL SHOWEES. 169 

by attributing it to the stoppage in the solar atmosphere of a succession 
of meteoric bodies flying toward the Sun with an enormous velocity, 
accelerated by the solar attraction until they enter this atmosphere, 
when their force is extinguished by its resistance. 

314. Size and Distance from the Earth. — All the parti- 
cles which compose the November shower are small; it 
has been estimated that some of them weigh but two 
grains, and that comparatively few exceed a pound. They 
begin to burn at a height of 74 miles, and are burnt up 
and disappear at an elevation of 54 miles; the average 
length of their visible paths being 42 miles. It is sup- 
posed that the November-shower meteors are composed 
of more easily destructible or more inflammable materials 
than aerolitic bodies. 

315. Other Star-showers, — What has been said about 
the appearance of the November meteors applies to the 
other star-showers, particularly those of August and 
April, the meteors of which also travel round the Sun in 
cometary orbits. In fact, there is reason to believe that 
three bodies which were observed and recorded as comets, 
were really nothing but meteors, and belonged one to the 
November, one to the August, and one to the April group. 

The August Meteors appear about the 10th of that 
month, their radiant-point being in the constellation Per- 
seus, and their number inferior to that of the November 
meteors. It is believed that they are distributed, though 
in unequal numbers, along their entire orbit, and that this 
orbit is considerably inclined to the plane of the ecliptic, 
and extends beyond the orbit of Neptune. 

3 1 6. Detonating Meteors. — In the case of the Novem- 
ber and August meteors and shooting-stars generally, the 



supply of solar heat ? 314. What is the size of the particles that compose the 
November shower? What is their height? Of what kind of materials are they 
thought to he composed? 315. To what else will what has heen said about the 
November meteors apply? What is there reason to believe respecting three 
bodies that passed for comets ? When do the August meteors appear ? Where is 
their radiant-point? How are they thought to be distributed? What is said of 



170 METEOES AND METEOEITES. 

mass is so small that it is entirely changed into vapor and 
disappears without noise. There are other classes of 
meteoric bodies, however, with much more striking 
effects. At times meteors of unusual brilliancy are heard 
to explode with great noise ; these are called Detonating 
Meteors. On November 15th, 1859, a meteor of this 
class passed over New Jersey ; it was visible in the full 
sunlight, and was followed by a series of terrific ex- 




Fig. 69.— Fire-ball, as observed in a Telescope. 

plosions, which were compared to the discharge of a 
thousand cannon. Other meteors are so large that they 
reach the Earth before complete vaporization takes place ; 
and we then have a fall of what are called Meteorites, 
often accompanied by loud explosions. 

317. Meteorites. — Meteorites are masses which, owing 
to their size, resist the action of the atmosphere, and 
actually complete their fall to the Earth. They are 
divided into Aerolites, or meteoric stones ; Aerosiderites, 
or meteoric iron / and Aerosiderolites, which comprise 
the intervening varieties. 

318. We do not know whether the meteorites, and 
meteors which occasionally appear, and which are there- 



their orbit ? 316. Why do shooting-stars generally disappear without noise ? 
What other meteoric bodies are there with which this is not the case ? Describe 
a detonating meteor that passed over New Jersey. What happens in the case of 
very large meteors ? 317. What are Meteorites? Into what classes are they di- 



SHOWEES OF AEEOLITES. 1*71 

fore called Sporadic Meteors, belong to groups or not, 
although, like the star-showers, they are most common 
at particular dates. As they are independent of geo- 
graphical position, it has been thought that there may 
be some astronomical and perhaps a physical difference 
between them and the ordinary shooting-stars. 

319. Showers of Aerolites. — Meteorites of considerable 
size occasionally reach the Earth's surface, by which men 
and cattle have been killed, and buildings set on fire. 
They sometimes fall in showers. 

Among the largest aerolitic falls of modern times, we 
may mention the following. On the 26th of April, 1803, 
at 2 p. m., a violent explosion was heard at L'Aigle, in 
Normandy, a luminous meteor having appeared in the air 
with a very rapid motion a few moments before. Two 
thousand stones fell, so hot as to burn the hands when 
touched. The shower extended over an area nine miles 
long and six miles wide, close to one extremity of which 
the largest of the stones, weighing nearly twenty-four 
pounds, was found. 

A similar shower of stones fell at Stannern, between 
Vienna and Prague, on the 22d of May, 1812, when two 
hundred stones fell on an area eight miles long by four 
miles wide. The largest stones in this case were found, 
as before, near the northern extremity of the ellipse. A 
third stone-fall occurred at Orgueil, in the south of France, 
on the evening of the 14th of May, 1864. The area over 
which the stones were scattered was eighteen miles long 
by five miles wide. 

At Kuyahinza, in Hungary, on the 9th of June, 1866, 
a luminous meteor was seen, and an aerolite weighing six 
hundredweight, and nearly one thousand smaller stones, 
fell on an area measuring ten miles in length and four in 
width. The large mass was found, as in the other cases, 

vided? 318. What is meant by Sporadic Meteors ? What has been thought re- 
specting them ? 319. Describe the fall of aerolites at L'Aigle. At Stannern. At Or- 



172 METEOES AND METEOEITES. 

at one extremity of the oval area. The fall was followed 
by a loud explosion, and a smoky streak was visible in the 
sky for nearly half an hour. 

320. Chemical Composition of Meteorites. — A chemical 
examination of the fragments of meteorites shows that, 
although in their composition they are unlike any other 
natural product, their elements are all known to us, and 
that they consist of the same materials, though in each 
variety some particular element may predominate. In the 
main, they are composed of metallic iron and various com- 
pounds of silica, the iron forming as much as 95 per cent, 
in some cases, and only 1 per cent, in others ; hence the 
division into the three classes referred to in Art. 317. 
The iron is always associated with a certain quantity of 
nickel, and sometimes with cobalt, copper, tin, and chro- 
mium. Among the silicates may be mentioned augite, 
and olivine, a mineral found abundantly in volcanic rocks. 

Besides these substances, a compound of iron, phospho- 
rus, and nickel, called sehreiberzite, is generally found ; this 
compound is not a natural terrestrial product, but has been 
artificially produced. Carbon has also been detected. 

321. The chemical elements found in meteorites up to 
the present time are as follows : — 

Metalloids: — Oxygen, sulphur, phosphorus, carbon, 
silicon. 

Metals : — Iron, nickel, chromium, tin, aluminum, mag- 
nesium, calcium, potassium, sodium, cobalt, manganese, 
copper, titanium, lead, lithium, strontium. 

322. Structure of Meteorites. — The structure of meteor- 
ites having been carefully studied with the microscope, it 
has been ascertained that the matter of which they are 
composed was certainly at one time in a state of fusion ; 



gueil. At Kuyahinza. 320. What is shown by a chemical examination of the frag- 
ments of meteorites ? Of what are they composed in the main ? What are found 
among the silicates ? What is said of sehreiberzite ? 321. Which of the metalloids 
have been found in meteorites ? Which of the metals ? 322. What has been ascer- 



STKUCTUKE OF METEOKITES. 1*7 3 

and that the most remote condition of which we have posi- 
tive evidence was that of small, detached, melted globules. 
The formation of these cannot be satisfactorily explained 
except by supposing that their constituents were originally 
in the state of vapor, as they now exist in the atmos- 
phere of the Sun ; and that, on the temperature's becoming 
lower, they condensed into these " ultimate cosmical parti- 
cles." These afterward collected into larger masses, which 
have been variously changed by subsequent metamorphic 
action, broken up by repeated mutual impact, and often 
again collected together and solidified. The meteoric irons 
are probably those portions of the metallic constituents 
which were separated from the rest by fusion, when the 
metamorphism was carried to that extreme point. 



CHAPTER XII. 

APPARENT MOVEMENTS OF THE HEAVENLY 
BODIES. 

323. In the preceding chapters we have studied in de- 
tail the whole universe of which we form a part; its 
nebulae and stars ; the nearest star to us — the Sun ; and 
lastly, the system of bodies which centre in this star, our 
own Earth being among them. 

We should now, therefore, be in a position to see ex- 
actly what " the Earth's place in Nature " really is. We 
find it, in fact, to be a small planet travelling round a 
small star, and that the whole solar system is but a mere 
speck in the universe — an atom of sand on the shore, a 
drop in the infinite ocean of space. 

tained by a microscopic examination of the structure of meteorites ? How is 
the formation of these detached melted globules explained ? What are the mete- 
oric irons supposed to be ? 

323. What have we studied in the preceding chapters? What, therefore, 
should we now be able to see ? What in fact is the Earth, and what the whole 



174 APPARENT MOVEMENTS 

324. The Earth an Observatory. — But, however unim- 
portant the Earth may be, compared with the universe 
generally, or even with the Sun, it is all in all to us who 
inhabit it, and especially so in an astronomical light; 
for, although we have in imagination looked at the vari- 
ous celestial orbs from all points of view, our bodily eyes 
are chained to the Earth. The Earth is, in fact, our ob- 
servatory, the very centre of the visible creation ; and this 
is why, until men knew better, it was thought to be the 
very centre of the actual one. 

More than this, the Earth is not a fixed observatory ; 
it is a movable one, and has a double motion, turning 
round its own axis while it travels round the Sun. Hence, 
although the stars and the Sun are at rest, they appear to 
us to move rapidly, and rise and set every twenty-four 
hours. Though the planets go round the Sun, their circu- 
lar movements are not visible to us as such, for our own 
annual movement is mixed up with them. 

Having described the heavens, then, as they are, we 
must describe them as they seem. The real movements 
must now give way to the apparent ones ; we must, in 
short, take the motion of our observatory, the Earth, into 
account. 

325. Apparent Movements, how prodnced. — To make 
this matter clear, let the Earth be supposed to be at rest, 
neither turning on its axis nor travelling round the Sun. 
In that case, the side turned toward the Sun would have 
perpetual day, the other side perpetual night. On the 
one side, the Sun would appear at rest — there would be 
no rising and setting ; on the other, the stars would be 
seen at rest in the same manner. The whole heavens 
would be, as it were, dead. 

solar system ? 324. What makes the Earth important to us, in an astronomical 
point of view ? What kind of an observatory is the Earth ? What apparent mo- 
tions are the result of the Earth's real motions ? Why do not the planets appear 
to move in circles ? What movements are we now to consider ? 325. If the 
Earth had no motion at all, what would follow ? If the Earth turned on its axis 



OP THE HEAVENLY BODIES. 17 5 

Again, let us suppose the Earth to go round the Sun 
as the Moon goes round the Earth, turning once on its 
axis during each revolution, which would result in the 
same side of the Earth always being turned toward the 
Sun. The inhabitants of the illuminated hemisphere 
would, as before, see the Sun motionless in the heavens ; 
but in this case, those on the other side, although they 
would never see the Sun, would still see the stars rise and 
set once a year. 

These examples show how the Earth's real movements 
produce the various apparent movements of the heavenly 
bodies. The latter are mainly of two kinds — Daily Ap- 
parent Movements and Yearly Apparent Movements ; the 
first being due to the Earth's daily rotation on its axis, 
and the second to the Earth's yearly revolution round the 
Sun. In each case the apparent movement is, as it were, 
a reflection of the real one, in the opposite direction ; ex- 
actly what one observes when travelling smoothly in a 
railroad train or balloon. When we travel in an express 
train, the objects appear to fly past us in the opposite di- 
rection to that in which we are going ; and to the occu- 
pants of a balloon, in which not the least sensation of mo- 
tion is felt, the Earth always seems to fall down from 
them, or rush up to meet them, when the balloon rises or 
descends. 

The Celestial Sphere. 

326. The Celestial Sphere.— We shall first study the 
effects of the Earth's rotation on the apparent movements 
of the stars. 

The daily motion of the Earth is very different in dif- 
ferent parts — at the equator and poles, for instance. An 

but once during each revolution round the Sun, what would be the result ? What 
should these examples show us ? Into what classes may the apparent move- 
ments be divided ? Of what is the apparent movement in each case a reflection ? 
Illustrate this. 326. How does the daily motion of the Earth differ in different 
parts ? What should we expect to see in consequence ? What do we see ? What 



176 THE CELESTIAL SPHEEE. 

observer at a pole is simply turned round without chan- 
ging his place, while one at the equator is swung round 
a distance of nearly 25,000 miles every day. We ought, 
therefore, to expect to see corresponding differences in the 
apparent motions of the heavens, if they are really due to 
the actual motions of our planet. Now, this is exactly 
what is observed. Not only is the apparent motion of the 
heavens from east to west — the real motion of the Earth 
being from west to east — but those parts of the heavens 
which are over the poles appear at rest, while those over the 
equator appear in most rapid motion. In short, the appar- 
ent motion of the Celestial Sphere — the name given to the 
visible vault of the sky — to which the stars appear to 
be fixed, and to which their positions are always referred, 
is exactly similar to the real motion of the terrestrial 
sphere, our Earth ; but, as we said before, in an opposite 
direction. 

Before proceeding further, however, we must explain 
the terms applied to different parts of the Celestial Sphere. 

327. Celestial Poles and Equator ; Zenith and Nadir. — 
In the first place, as the stars are so far off, we may im- 
agine the centre of the Celestial Sphere to lie either at the 
centre of the Earth or in our eye, and we may imagine it 
as large or as small as we please. The points at which 
the Earth's axis would pierce this sphere, if it were ex- 
tended at each end, we call the Celestial Poles ; the great 
circle lying in the same plane as the terrestrial equator is 
distinguished as the Celestial Equator, or Equinoctial. 
The point overhead is the Zenith ; the point beneath our 
feet, the Nadir. 

328. Declination and Eight Ascension. — As the Earth 
is belted by parallels of latitude and meridians of longi- 



is the Celestial Sphere ? To what is the apparent motion of the celestial sphere 
similar ? 327. Where may we imagine the centre of the celestial sphere to lie ? 
What is meant by the Celestial Poles? What is the Celestial Equator? The 
Zenith? The Nadir? 328. By what are the heavens belted, to the astronomer? 



DECLINATION AND EIGHT ASCENSION. 177 

tude, so are the heavens belted to the astronomer with 
parallels of declination and meridians of right ascension. 

If we suppose the plane in which our equator lies, ex- 
tended to the stars, it will pass through all the points 
which have no declination (0°). Above and below this 
plane we have north and south declination, as we have 
north and south latitude, till we reach the pole of the 
equator (90°). 

As we start from the meridian of Greenwich in reckon- 
ing longitude, so do we start from a certain point in the 
celestial equator occupied by the Sun at the vernal equi- 
nox, called the first point of Aries, in measuring right 
ascension. As we say such a place is so many degrees 
east of Greenwich, so we say such a star is so many hours, 
minutes, or seconds, east of the first point of Aries. 

In short, as we define the position of a place on the 
Earth by saying that its latitude and longitude (in de- 
grees) are so-and-so, in like manner do we define the posi- 
tion of a heavenly body by saying that, referred to the 
celestial sphere, its declination (in degrees) and right 
ascension (in time reckoned from Aries) are so-and-so. 

329. Sometimes the distance from the north celestial 
pole is given instead of that from the celestial equator. 
This is called North-polar Distance, and is denoted in 
almanacs, etc., by the initials 1ST. P. D. As the pole is 90° 
from the equator, the north-polar distance is evidently the 
complement of the declination — that is, the difference be- 
tween the declination and 90°. 

330. The Horizon. — Altitude, Azimuth, etc. — The terms 
defined above apply to the celestial sphere generally. 
When we consider that portion of it visible in any one 

To what on the Earth do these lines correspond? What in the heavens corre- 
spond to latitude and longitude on the Earth ? From what is declination meas- 
ured ? From what, right ascension ? Illustrate the way in which these terms 
are used. 339. To locate a point in the heavens, what is sometimes given instead 
of its declination ? What relation do the north-polar distance and the declina- 
tion bear to each other ? 330. What is meant by the Visible or Sensible Horizon ? 



178 ALTITUDE AND AZIMUTH. 

place, or the sphere of observation, there are other terms 
employed, which we proceed to explain. 

In any place the visible portion of the celestial sphere 
seems to rest on either the Earth or the sea. The line 
where the heavens and Earth seem to meet is called the 
Visible or Sensible Horizon; the Plane of the Visible 
Horizon meets the Earth at that point of the surface 
which is occupied by the observer. The Rational, or True 
Horizon, is a great circle of the heavens, the plane of 
which is parallel to the former plane, but which, instead 
of being a tangent to tho Earth's surface, passes through 
its centre. 

331. A Vertical Line is a line passing from the zenith 
to the nadir, and therefore through the observer; it is 
clearly at right angles to the planes of the horizon. 

332. If it is desired to point out the position of a 
heavenly body, not on the celestial sphere generally, but 
on that portion of it visible above the horizon of a place 
at a given moment, this is done by determining either its 
altitude or zenith-distance, and its azimuth (instead of its 
declination and right ascension). 

Altitude is the angular height above the horizon. 

Zenith-distance is the angular distance from the zenith. 
As the zenith is 90° from the horizon, the zenith-distance 
is evidently the complement of the altitude — that is, the 
difference between the altitude and 90°. 

Azimuth is the angular distance between two planes, 
one of which passes through the north or south point 
(according to the hemisphere in which the observation is 
made), and the other through the object, both passing 
through the zenith. Azimuth is to Altitude what longi- 

Where does the Plane of the Visible Horizon touch the Earth ? What is the Ea- 
tional or True Horizon ? 331. What is a Vertical Line ? 333. How is the posi- 
tion of a heavenly body pointed out on that part of the celestial sphere which is 
visible above the horizon of a place at a given moment? What is Altitude ? 
What is Zenith-distance ? What relation do they bear to each other? What is 
Azimuth ? What relation does azimuth bear to altitude ? 333. What is meant 



APPAEENT MOVEMENTS OF THE STAES. 179 

tude is to latitude, or what right ascension is to declina- 
tion. 

333. The Celestial Meridian of any place is the great 
circle on the sphere corresponding to the terrestrial 
meridian of that place, cutting therefore the north and 
south points. 

The Prime Vertical is another great circle passing 
through the east and west points and the zenith. 

Apparent Movements of the Stars. 

334. Rising, Culmination, and Setting of the Stars. — 
We are now prepared to proceed with our inquiry into 
the apparent movements of the celestial sphere. We shall 
continue to speak of the Sun or a star as rising or setting, 
although the student now understands that it is, in fact, 
the plane of the observer's horizon which changes its 
direction with regard to the heavenly body, in consequence 
of its being carried round by the Earth's motion. When 
a star is so situated that it is just visible on the eastern 
horizon, it is said to rise. When the rotation of the 
Earth has brought the plane of the horizon under the 
meridian which passes through the star, the latter is said 
to culminate or pass the meridian. When the plane of 
the horizon is carried to the nadir of the point it passed 
through when the star first became visible, the star 
appears on the opposite — that is, the western— horizon, 
and is said to set. 

335. Apparent Movements, as &un from Different Parts 
of the Earth. — Let Fig. 10 represent the celestial sphere, 
and N an observer at the north pole of the Earth. To 
him the north pole of the heavens (P) and the zenith 
(Z) coincide, and his true horizon is the celestial equa- 



by the Celestial Meridian of any place ? What is the Prime Vertical ? 334. When 
we speak of the Sun or a star as rising or setting, what do we really mean ? When 
is a star said to culminate? 335. With Fig. 70, explain the apparent movements 



180 



APPAKENT MOVEMENTS OF THE STAKS. 



zi> 



tor. Above his head is the pivot on which the heavens 
appear to revolve, as beneath his feet is the pivot on which 

the Earth actually revolves; 
and round this point the stars 
appear to move in - circles, 
which get larger and larger 
as the horizon is approached. 
The stars never rise or set, 
Moriz^ 11 ^ J but always keep the same dis- 
tance from the horizon. The 
observer is merely carried 
round by the Earth's rota- 
tion, and the stars seem to be 

Fig. 70. — Celestial Spheke, viewed • -> t • . i •■ 

PROM THE POLES. A PARALLEL CMIM* TOUUd 1U the OppOSlte 




direction. 

336. We will now change our position. In Fig. 71 
the celestial sphere is again represented, but this time we 
suppose an observer, Q, at its centre, to be on the Earth's 

equator. In this position 
we find the celestial equa- 
tor in the zenith, and the 
celestial poles PP on the 
true horizon, and the stars, 
instead of revolving round 
a fixed point overhead, and 
never rising or setting, rise 
and set every twelve hours, 
travelling straight up and 
down along circles which 
Fig. 71.— The Celestial Sphere, viewed get Smaller and smaller as 

PROM THE EQUATOR. A EIGHT SPHERE. ^ ^^ ^ ^^ ^ 

approach the poles. The spectator is carried up and down 
by the Earth's rotation, and the stars appear to be so carried. 



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7 


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i 


Z 

J" 


j 


/t 


1 1 


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4. 



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of the stars, as seen from the north pole. 336. How is the ohserver supposed to 
be placed in Fig. 71 ? In this position, where do we find the celestial equator, 
and where the poles ? How do the stars appear to move, and why ? 337. In Fig. 



AN OBLIQUE SPHERE. 



181 





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337. Yet another figure, to show what happens half- 
way between the poles and the equator. At O, in Fig. 
72, we imagine an observer to be placed on our Earth in 
latitude 45° (that is, half-way between the equator in 
lat. 0°, and the north pole in lat. 90°). Here the north 

celestial pole will be 
half-way between the 
zenith and the hori- 
zon (see Figs. 70 and 
71) ; and close to the 
pole he will see the 
stars describing cir- 
cles, inclined, how- 
ever, and not retain- 
ing the same dis- 
tance from the hori- 
zon. As the eye 
leaves the pole, the 

Fig. 72.-Celestial Sphere, viewed prom a Mid- Sta ^ S nSe ancl Set 
dle Latitude. An Oblique Sphere. DB' rep- obliquely, and de- 
resents the apparent path of a circumpolar star ; ., , . , 
B B' B", the path and rising and setting points SCriDe larger Circles, 
of an equatorial star ; C C C" and A A' A", those praduallv dinning 
of stars of mid-declination, one north and the ° •> ^" ° 

other south. more and more un- 

der the horizon, until, when the celestial equator, B B' B" , 
is reached, half their journey is performed below it. 
Farther south, we find the sfcars rising less and less above 
the horizon ; and finally, as there were northern stars that 
never dip below the horizon, so there are southern stars 
which never appear above it. 

To observers in lat. 45° south, the southern celestial 
pole is in like manner visible ; the stars we never see in 
the northern hemisphere, never set ; the stars which never 
set with us, never rise ; the stars which rise and set with 
us, set and rise with them. 

72, where do we suppose the observer to be placed? Explain the movements of 
the stars, as seen from this point. What appearances would be presented in lat. 



JST 



182 APPAKENT MOVEMENTS OF THE STABS. 

338. Stars visible in Different Latitudes. — Now let the 
celestial sphere be divided into two hemispheres, a northern 
and a southern : it is evident that an observer at the north 
pole sees only the stars of the northern hemisphere ; one 
at the south pole, only those of the southern ; while one at 
the equator sees both. An observer in a middle north 
latitude sees all the northern stars and some of the 
southern ones ; and another in a middle southern latitude 
sees all the southern stars and some of the northern ones. 

Hence, in middle latitudes, and therefore in the United 
States, we may divide the stars into three classes :— 

I. Those northern stars which never set (northern 

circumpolar stars). 
II. Those southern stars which never rise (southern 

circumpolar stars). 
III. Those stars which both rise and set. 

339. It is easily gathered from Figs. 70, 71, 72, that the 
height of the celestial pole above the horizon at any place 
is equal to the latitude of that place ; for at the equator, 
in lat. 0°, the pole was on the horizon, and consequently 
its altitude was nothing ; at the pole, in lat. 90°, it was in 
the zenith, and its altitude was therefore 90° ; while in 
lat. 45° its altitude was 45°. Accordingly, at New York, 
in lat. 40f °, its altitude will be 40f ° ; hence, stars of less 
than that distance from the pole will always be visible, as 
they will be above the horizon when passing below the 
pole. All the stars, therefore, within 40f° of the north 
pole will form Class I. ; all those within 40f ° of the south 
pole, Class II. ; and the remainder — that is, all stars from 
49J° K (90 o -40f°=49j°) to 49j S. will form Class III. 

45° south? 338. What stars are visible to an observer at the north pole ? At the 
south pole ? At the equator ? In a middle northern latitude ? In a middle 
southern latitude ? In middle latitudes, how may the stars he divided ? 339. 
What is the height of the celestial pole above the horizon at any place ? Illus- 
trate this in the case of New York, and state what stars will there belong to 
each of the three classes just specified. What stars belong to each of the three 
classes, in the latitude in which you live ? 340. What may be used with advan- 



USE OF THE GLOBES. 183 

340. Use of the Globes. — In these and similar inquiries 
the use of the terrestrial and the celestial globe is of great 
importance. To use either properly, we must begin by- 
making each a counterpart of what is represented; that 
is, the north pole must be north, the south pole south, and 
the axis of either globe must be made parallel to the 
Earth's axis. To find the north point, a compass may be 
used, allowance being made for its variation from the 
true meridian, as established for the time and place in 
question. 

The brazen meridian being thus made to run due 
north and south, the pole — the north pole in our case — 
must be elevated to correspond with the latitude of the 
place where the globe is used. At the poles this would 
be 90°, at the equator 0°, and at New York 40f °. The 
wooden horizon, if perfectly level, will then represent the 
true horizon of the place. 

If we now turn the terrestrial globe from west to 
east, we exactly represent the Earth's motion ; and, turn- 
ing the celestial globe from east to west, we have an exact 
representation of the apparent movements of the stars 
for the place in question. It will be seen that some stars 
never descend below the true horizon, while others never 
rise above it. 

341. Position of the North Celestial Pole. — At the 
present time the north celestial pole lies in Ursa Minor, 
and a star in that constellation very nearly marks the 
position of the pole, and is therefore called Polaris, or the 
Pole-star. The direction in which the Earth's axis points 
is not always the same, although it varies so slowly that 
a few years do not make much difference. As a conse- 
quence, the position of the celestial poles, which are the 



tage in these inquiries ? How are the globes prepared for use ? How may the 
Earth's motion and the apparent movements of the stars be exactly represented ? 
341. Where is the north celestial pole at present situated ? What star marks its 
position ? What causes change in the position of the celestial poles ? 342. What 



184 APPAEENT MOVEMENTS OF THE STAES. 

points where the Earth's axis prolonged would strike the 
celestial sphere, varies also. 

342. The Circumpolar Constellations. — One of the most 
striking northern circumpolar constellations is Ursa Major 
or the Great Bear (see Fig. 17, p. 30), also called Charles's 
Wain and the Plough. Seven bright stars in this con- 
stellation (connected by lines in Fig. 17) form what is 
called the Great Dipper, three making the handle and 
four the bowl. The two stars of the bowl which are 
farthest from the handle (Merak and Dubhe) are called the 
Pointers, because the straight line which connects them 
points very nearly to the north pole, in whatever position 
the constellation may be. This will be seen from Fig. 74. 

The Pole-star, readily found with the aid of the Point- 
ers, is at the extremity of the tail of the Little Bear, and 
unites with six other stars of that constellation to form 




Fig. 73.— The Northern Circumpolar Constellations. 

the Little Dipper (see Fig. 73). This resembles the 

is one of the most striking of the northern circumpolar constellations ? 
How is the Great Dipper formed? What stars are called the Pointers, 
and why? In what constellation is the Pole-star? What does it help to 
form? Name the other more important northern circumpolar constellations. 
What is Cassiopea sometimes called, and why? 343. What are the principal 



CIKCUMPOLAE CONSTELLATIONS. 



185 



Great Dipper in shape, but is smaller and formed of less 
brilliant stars. 

The other more important northern circumpolar con- 
stellations are Cassiopea, Cepheus, Camelopardalus, and 
Draco. Seven stars in Cassiopea form what may be fan* 
cied to be the outline of a chair (see Fig. 73), and hence 
this constellation is sometimes called the Lady's Chair. 

343. The principal southern circumpolar constellations 
are Crux (the Cross), Triangulum Australe (the Southern 
Triangle), Ara (the Altar), Pavo (the Peacock), Toucanus 
(the Toucan), Hydrus (the Water-snake), and Dorado (the. 
Sword-fish). All these can be found on the Celestial Chart 
of the Southern Hemisphere. The other constellations 
mentioned in Arts. 55, 56, and 57, belong to Class III., 
both rising and setting to observers in the United States. 

344. Period of the Apparent Movements of the Celes- 
tial Sphere. — As 
the Earth's ro- 
tation is accom- 
plished in 23h. 
56 m. 4 s., it fol- 
lows that the ap- 
parent move- 
ment of the ce- 
lestial sphere is 
completed in that 
time; and were 
there no clouds, 
and no Sun to 
eclipse the stars 
in the daytime 
by his superior 
brightness, we 











I 




hII 


wan 






■fl 


Hehb 






^H 












. - 


i&uSiil 


IfenHfi 








in 


Hi 


ram 

m 

m 
m 


n 1 

■ 

1 














^^RHf? 




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^^f^f 



Fig. 



U— Different Positions of the Great Dipper 
at Intervals of Six Hours. 



southern circumpolar constellations ? To what class do the other constella- 
tions belong? 344. In what time is the apparent movement of the celestial 
sphere completed, and why? If there were no clouds, Avhat should we 



186 APPAEENT MOVEMENTS OF THE STAKS. 

should see the grand procession of distant worlds ever de- 
filing before us, and commencing afresh after that period. 
The circumpolar constellations would be seen to make a 
complete revolution round the Pole-star. The Great Dip- 
per, for instance, would at intervals of six hours occupy 
the different positions shown in Fig. 74. 

345. Effect of the Earth's Yearly Revolution on the 
Apparent Movements of the Stars. — We see stars only at 
night, because in the daytime the Sun puts them out ; and 
the particular stars we see on any given night are those 
which occupy that half of the celestial sphere opposite to 
the Sun. Now, as we go round the Sun, we are at differ- 
ent times on different sides, so to speak, of the Sun ; and 
if we could see the stars beyond him, we should see them 
change. But what we cannot do at mid-day, in conse- 
quence of the Sun's brightness, we can easily do at mid- 
night ; for, if the stars behind the Sun change, the stars 
exactly opposite to his apparent place will change too, 
and these we can see in the south at midnight. 

It is clear, in fact, that in one complete revolution of 
the Earth round the Sun every portion of the visible celes- 
tial sphere will in turn be exposed to view in the south at 
midnight. As the revolution is completed in 365 days, 
and there are 360° in a great circle of the sphere, we may 
say that the portion of the heavens visible in the south at 
midnight advances about 1° from night to night. This 1° 
is passed over in 4 minutes, as the whole 360° are passed 
over in nearly 24 hours. 

346. This advance is a consequence of the difference 
between the lengths of the day as measured by the fixed 
stars and by the moving Sun. As the solar day is longer 
than the sidereal day, the stars by which the latter is 



Bee ? What would the circumpolar constellations he seen to do ? 345. What 
other change is seen in the heavens, and to what is it due ? At what rate does 
the portion of the heavens visible in the south at midnight advance, aDd why ? 
346. Of what is this advance a consequence ? How do the solar and the sidereal 



HOW TO IDENTIFY THE STAKS. 187 

measured gain upon the former at the rate we have 
mentioned ; so that, as seen at the same hour on successive 
nights, the whole celestial vault advances to the west- 
ward, the change due to one month's apparent yearly mo- 
tion being equal to that brought about in two hours by 
the apparent daily motion. 

Hence the stars south at midnight (or opposite the 
Sun's place) on any night, were south at 2 a. m. a month 
previously, and so on ; and will be south a month hence 
at 10 o'clock p. m., and so on. 

347. How to Identify the Stars in the Sky. — A knowl- 
edge of the various stars and constellations may be ob- 
tained with the aid of a celestial globe.* We first, as 
already stated, place its brass meridian in the plane of 
the meridian of the place in which the globe is used, and 
make the axis of the globe parallel to the axis of the 
Earth and the heavens, by elevating the north pole until 
its height above the wooden horizon is equal to the lati- 
tude of the place. We next bring under the brazen me- 
ridian the actual place in the heavens occupied by the Sun 
at the time ; this place is given for every day 'in the alma- 
nacs. We thus represent exactly the position of the 
heavens at mid-day, and the index is then set at 1 2 ; for 
it is always '12, or noon, at a place, when the Sun is in the 
meridian of that place. Then, if the time at the place is 

* Whitall's Movable Planisphere, widely known and used in this country, 
is undoubtedly superior to any celestial globe or map for imparting an accurate 
acquaintance with the stars. This ingenious instrument, set for a given day of 
the month and a given hour and minute, held with the zenith overhead, and the 
extremity of the meridian marked north toward the north, shows the constella- 
tions and principal stars visible at the time (and only these) in the exact posi- 
tions which they then occupy in the heavens, so that they can be distinguished 
and named with the utmost ease. A variety of problems may be solved with the 
Plauisphere, the use of which invests the study with a practical interest which is 
truly surprising.— American Editor. 



day compare in length? In how long a time will the daily motion produce 
changes equal to those produced by one month's yearly motion? What follows 
with respect to the stars south at midnight on any given night? 347. How may 
a knowledge of the stars be obtained ? How is the globe rectified ? When it is 



188 USE OF THE CELESTIAL GLOBE. 

after noon, we turn the globe from east to west — if before 
noon, from west to east — till the index and the time cor- 
respond. 

When the globe has thus been rectified, as it is called, 
we have the constellations which are rising on the eastern 
horizon, just appearing above the eastern part of the 
wooden horizon. Those setting are similarly near the 
western part of the wooden horizon. The constellations 
in the zenith at the time will occupy the highest part of 
the globe, while the constellations actually on the merid- 
ian will be underneath the brazen meridian. 

348. Further, it is easy at once to see at what time any 
stars will rise, culminate, or set, when the globe is recti- 
fied in this manner. All that is necessary is, as before, to 
bring the Sun's place, given in the almanac, to the me- 
ridian, and set the index to 12. To find the time at 
which any star rises, we bring it to the eastern edge of 
the wooden horizon, and note the time, which is the time 
of rising. To find the time at which any star sets, we 
bring it similarly to the western edge of the wooden hori- 
zon and note the time, which is the time of setting. To 
find the time at which any star culminates, we bring the 
star under the brazen meridian and note the time, which 
is the time of meridian passage. 

349. In the absence of the celestial globe [or plani- 
sphere], the student will derive assistance from the follow- 
ing table, which indicates the positions occupied by the 
constellations at certain hours during each month in the 
year. The constellations should be looked for in the sky 
in the positions specified, and compared with the Celestial 
Chart at the end of the volume, where the names of the 
bright stars they contain will be found. The stars in 

thus reclined, where are the constellations rising, setting, and at the zenith, 
respectively seen ? 348. How can it be found when a given star will rise, set, or 
culminate ? 349. What is furnished, to aid the student in finding the stars and 
constellations, in the absence of the celestial globe or planisphere ? What sug- 
gestions are made as to the use of the table ? 



CONSTELLATIONS VISIBLE IN THE U. S. 189 

their vicinity may then be traced. A little study and a 
few comparisons of the heavens with the charts will soon 
familiarize the pupil with the principal stars and constella- 
tions, and their relative positions. 

CONSTELLATIONS VISIBLE IN THE UNITED STATES ON 
DIFFERENT EVENINGS THROUGHOUT THE YEAR.* 

Jan. 20, 10 p.m. 
(Feb. 4, 9 p.m. ; Feb. 19, 8 p.m. ; Dec. 21, midn't; Jan. 5, 11 p.m.) 
N — S. Draco, polaris, Oamelopardalus, *Auriga, Orion, Lepus, 

Columba Noachi. 
E — W. Leo, Cancer, * Aries, Cetus. 
KE— SW. Canes Yenatici, Ursa Major, Lynx, * Taurus, Eridanus. 
SE — NW. Monoceros, Gemini, „, Perseus, Andromeda. 

Look for Capella a little west or north-west of the zenith ; 
and Betelgeuse, or a Orionis, very near the meridian, about one- 
third of the way from the zenith to the southern horizon. A 
little west of Betelgeuse is Bellatrix. 

Feb. 20, 9£ p.m. 
(Feb. 27, 9 p. m. ; Mar. 15, 8p.m.; Jan. 13, midn't; Jan. 28, 11 p. m.) 
N — S. Draco, Ursa Minor, polaris, *Lynx, Gemini, Canis 

Minor, Monoceros, Ship Argo. 
E — W. Yirgo, Leo Minor, * Auriga, Taurus. 
NE — SW. Bootes, Canes Yenatici, Ursa Major, * Orion, Eridanus. 
SE — NW. Hydra, Leo, * Perseus, Andromeda. 

Look for Castor very near the zenith, a little to the west. 
Near it is Pollux, on the meridian, and in the zenith in lat. 31°. 
Procyon, in Canis Minor, is very nearly on the meridian, about 
one-third of the distance from the zenith to the southern horizon. 
The Milky Way will be seen running along the heavens, in a 
curve west of the meridian and not far distant from it, from the 
northern to the southern horizon. 

March 21, 10 p.m. 
(Apr. 5, 9 p. m. ; Apr. 20, 8 p. m. ; Feb. 19, midn't ; Mar. 6, 11 p. m.) 

* The asterisk on the several lines denotes that the zenith (in lat. 40°, to which 
the tahle particularly refers, though it will serve for any lat. in the IT. S.) sepa- 
rates the two constellations between which the asterisk is placed. When the as- 
terisk is prefixed to any constellation, the constellation itself occupies the zenith. 



190 CONSTELLATIONS VISIBLE IN THE U. S. 

N" — S. Cepheus, polaris, Ursa Major, * Leo Minor, Leo, Hydra. 
E — W. Virgo, Coma Berenices, * Gemini, Orion. 
NE — SW. Hercules, Corona Borealis, Bootes, Canes Venatici, * 

Cancer, Monoceros, Canis Major. 
SE — IW. Hydra, Virgo, * Lynx, Camelopardalus, Perseus. 

Look for Eegulus, the brightest star of the Lion, on the 
meridian, nearly one-third of the way from the zenith to the 
southern horizon. 

- April 20, 10 p.m. 
(May 5, 9 p. m. ; May 20, 8 p.m.; Mar. 20, midn't ; Apr. 5, 11 p. m.) 
N" — S. Cassiopea, Cepheus, polaris, Ursa Major, ^ Coma Bere- 
nices, Virgo, Corvus. 
E — W. Ophiuchus, Hercules, Corona Borealis, Bootes, Cor 
Caroli, * Leo Minor, Cancer, Canis Minor, Mono- 
ceros. 
NE — SW. Lyra, Draco, * Leo Minor, Leo, a Hydrm. 
SE — NW. Libra, Bootes, * Lynx, Auriga. 

Look for Denebola, or /3 Leonis, a short distance west of the 
meridian, and a little less than £ of the way from the zenith to 
the southern horizon. Alpha (a) Hydrm, in the south-west, 
otherwise called Cor Hydros and Alphard, is a variable star, 
ranging from the second to the third magnitude. It can be easily 
found from the fact that there is no other large star near it. 

Mat 21, 10 p.m. 
(June 5, 9 p. m. ; May 28, 9| p. m. ; May 6, 11 p. m. ; Apr. 20, midn't.) 
N — S. Cassiopea, polaris, v Ursm Majoris or AcTcair, * Arc- 

turus or a Bootis, Virgo, Centaurus. 
E — W. Aquila, Hercules, * Canes Venatici, Cor Caroli, Leo 
Minor, Leo, a Hydrm. 
NE — SW. Vulpecula et Anser, Lyra, Hercules, * Coma Berenices, 

Virgo, Crater. 
SE — NW. Scorpio, Ophiuchus, Serpens, Bootes, * Ursa Major, 
Lynx, Gemini. 

Look for Spica, or a Virginis, west of the meridian, and more 
than half-way from the zenith to the southern horizon. 

June 21, 10 p.m. 
(July 6, 9 p.m. ; June 29, 9| p. m. ; June 5, 11 p. m. ; May 22, midn't.) 
N — S. Camelopardalus, polaris, Ursa Minor, Draco, * Her- 
cules, Serpens, Scorpio. 



ON DIFFERENT EVENINGS OF THE YEAR. 191 

E — W. Delphinns, Vulpecula et Anser, Lyra, * Bootes, Canes 
Venatici, Coma Berenices, Leo. 
NE — SW. Pegasus, Cygnus, + Bootes, Virgo. 
SE — NW. Sagittarius, *Hercules, Ursa Major, Lynx. 

July 22, 10 p. m. 
(Aug. 5, 9 p. m. ; July 30, 9£ p. m. ; July 7, 11 p. m. ; June 22, midn't.) 
N — S. Oamelopardalus, polaris, Draco, * Lyra, Sagittarius. 
E — W. Pegasus, Oygnus, * Bootes, Yirgo. 
NE — SW. Andromeda, Cepheus, ^ Hercules, Serpens, Libra. 
SE — WW. Capricornus, Delphinus, Vulpecula et Anser, Lyra, 
* Ursa Major, Leo Minor. 
Look for the bright star a Lyra, otherwise called Vega and 
Lyra, very near the zenith, a little east of the meridian. North- 
east of Vega, and very near it, is e Lyra, the remarkable double- 
double star described in Art. 66. 

Auo. 23, 10 p. m. 
(Sept. 7, 9 p. m. ; Aug. 31, 9£ p. m. ; Aug. 8, 1 1p.m.; July 24, midn't.) 
N — S. Oamelopardalus, polaris, Cepheus, Draco, *Cygnns, 
Vulp. et Anser, Delphinus, Antinous, Capricornus. 
E — W. Pisces, a Andromeda or Alpheratz, # Lyra, Hercules, 
Serpens, Libra. 
NE — SW. Perseus, Andromeda, *Cygnus, Ophiuchus. 
SE — NW. Aquarius, Pegasus, * Hercules, Bootes, Canes Venatici. 
Nearly midway between the zenith and the eastern horizon, 
look for the Square of Pegasus, formed by four double stars of the 
second magnitude, Scheat and Markab at the western angles, and 
Alpheratz (a Andromeda) and Algenib (y Pegasi) at the eastern. 
The two stars last named are on the First Meridian, and with Caph 
(j3 Cassiopea), which is on the same line 30° north of Alpheratz, 
and Polaris, serve to define the position of the great circle from 
which right ascension is measured. 

Sept. 23, 10 p. m. 
(Oct. 16, 8£p. m. ; Oct. 1, 9i p. m. ; Sept. 7, H p. m. ; Aug. 24, midn't.) 
N — S. Ursa Major, polaris, Cepheus, * Pegasus, Aquarius, 

Piscis Australis. 
E — "W". Aries, Andromeda, * Cygnus, Ophiuchus. 
NE — SW. Auriga, Cassiopea, * Delphinus, Antinous, Sagittarius. 
SE — JSTW. Cetus, Pisces, a Andromedaa, * Cygnus, Hercules. 



192 CONSTELLATIONS VISIBLE IN THE U. S. 

Oct. 23, 10 p.m. 
(Nov. 7, 9 p. m. ; Nov. 23, 8 p. m. ; Sept. 23, midn't ; Oct. 8, 11 p.m.) 
N — S. Ursa Major, polaris, Oassiopea, * Andromeda, Pisces, 

Cetus. 
E — W. Orion, Taurus, Perseus, ^Pegasus, Delphinus, Antinous. 
NE — SW. Gemini, Auriga, Perseus, * Pegasus, Aquarius, Capri- 

cornus. 
SE — NW. Eridanus, Cetus, Aries, * Andromeda, Cygnus, Lyra, 
Hercules. 

The meridian now very nearly corresponds with the First 
Meridian. Caph, or (3 Cassiopece, will be found nearly due north 
of the zenith, and Alpheratz and Algenib a few degrees south of 
the zenith. Fomalhaut, a first-magnitude star in Piscis Australis, 
is on the meridian at 38 minutes past 8, Oct. 23, and may be 
seen near the southern horizon. 

Nov. 22, 10 p.m. 
(Dec. 7, 9p.m.; Dec. 23, 8 p.m.; Oct. 23, midn't ; Nov. 7, 11 p. m.) 
N — S. v Utscb Majoris, Draco, polaris, * Perseus, Trian- 
gulum, Aries, Cetus. 
E — W. Monoceros, Auriga, * Andromeda, Pegasus, Aquarius. 
NE — SW. Cancer, Lynx, ^ Pisces, Aquarius, Piscis Australis. 
SE — NW. Lepus, Orion, Taurus, ^ Andromeda, Cygnus. 

Look for the triple star Almaack, or y Andromeda?; very near 
the zenith ; in its neighborhood is an elliptical nebula, described 
in Art. 95. A little west of Almaack is a nebula of minute stars 
visible to the naked eye, supposed to be the nearest of all the 
great nebulae. The variable star Mira, described in Art. 74, is 
now almost on the meridian, about half-way between the zenith 
and the southern horizon. The interesting star Algol, or /? Persei 
(Art. 75), will be on the meridian, and at the zenith in lat. 40°, 
at 47 minutes past 10, Nov. 22. 

Dec. 21, 10 p.m. 
(Jan. 5, 9 p. m. ; Jan. 20, 8 p. m. ; Nov. 21, midn't ; Dec. 6, 11 p. m.) 
N — S. Draco, Ursa Minor, polaris, Camelopardalus, * Perseus, 

Taurus, Eridanus. 
E — W. Hydra, Cancer, Gemini, Auriga % * Triangulum, Pisces. 
NE — SW. Leo Minor, Ursa Major, Lynx, ^ Aries, Cetus. 
SE — NW. Canis Major, Monoceros, ^ Andromeda, Pegasus. 

About 15° south of the zenith, and a little west of the meridian, 



ON DIFFERENT EVENINGS OF THE YEAR. 



193 



is the group of the Pleiades (Art. 86), consisting of six stars visible 
to the naked eye, the brightest of which is Alcyone, of the third 
magnitude. South-east of the Pleiades 11°, and just east of the 
meridian, are the Hyades, a group which may readily be recog- 
nised by the brilliancy of its principal star, Aldebaran, or a Tauri. 
Aldebaran reaches the meridian at 25 minutes past 10, Dec. 21, 
or at 9 o'clock on Jan. 11. 

















• I 




H 


IBti 








.;'.'. 


H 


- 1 
















1 | 9 1 














HB 


'-"'-'' v " 9 | 






B^M 


Jm 


BhB 


SaaB 





Fig. 75.— Equatorial Constellations, visible in the South on 
Jan. 20, at 10 p. m. 

350. In Fig. 75 are shown some of the equatorial con- 
stellations visible in the south on the 20th of January. 
The central one is Orion, one of the most marked in the 
heavens. When all the bright stars in Orion are known, 
many of the surrounding ones may easily be found, by 
means of alignements. For instance, the line formed by 
the three stars in the belt, if produced in a south-easterly 
direction, will pass near Sirius, the brightest star in the 
heavens, and prolonged in the opposite direction will 
nearly pass through Aldebaran. Sirius, Betelgeuse, and 

350. What does Fig. 75 show ? What is said of Orion ? How may Sirius and 
Aldebaran he found ? What three bright stars form a triangle in this part of 
9 






194 EQUATORIAL CONSTELLATIONS. 

Procyon, form a triangle whose sides are nearly equal, 
Betelgeuse being at the westernmost, and Procyon at the 
easternmost, angle. 

351. Fig. 76 represents, in like manner, some of the 
equatorial constellations visible in the south on the 21st 
of May. Arcturus is now nearly on the meridian. East 
of it is the constellation Hercules, toward a point of which 
our Sun is travelling with his system of planets, satellites, 
and comets. Hercules may be recognized by the four- 
sided figure (nearly a square) formed by four of its 
brightest stars. 




Fig. 76. — Equatorial Constellations, visible in the South on 
May 21, at 10 p. m. 

Apparent Movements of the Sun. 

352. The effect of the Earth's daily movement upon 
the Sun is precisely similar to its effect on the stars ; that 
is, the Sun appears to rise and set every day. But in 

the heavens? 351. What does Fig. 76 represent? What star is now nearly on 
the horizon ? What constellation is east of Arcturus ? How may Hercules he 
recognized? 352. What is the effect of the Earth's daily movement on the Sun ? 
How is the Sun's apparent motion affected hy the Earth's yearly motion ? 353. 



SIDEREAL AND SOLAE DAY. 



195 



consequence of the Earth's yearly motion round it, it 
appears to revolve round the Earth more slowly than the 
stars ; and it is to this that we owe the difference between 
star-time and sun-time, or between the sidereal and the 
solar day. 

353. Difference between the Sidereal and the Solar 
Day. — How this difference arises is shown in Fig. 77, 
which represents the Sun, and the Earth in two positions 
in its orbit, separated by the time of a complete rotation. 
In the first position of the Earth are shown one observer, 
a, with the Sun on his meridian, and another, 5, with a 
star on his ; the two observers 
being on exactly opposite sides 
of the Earth, and on a line drawn 
through the centres of the Earth 
and Sun. In the second position, 
when the same star comes to 6's 
meridian, a sees the Sun to the 
east of his, and he must be car- 
ried by the Earth's rotation to 
c before the Sun occupies the 
same apparent position in the 
heavens that it did before — or 
is again on his meridian. The 
solar day, therefore, will be longer 
than the sidereal day by the time 
it takes a to travel this distance. 
Of course, were the Earth at 




Fig. 77.— Explanation of the 
Difference in Length be- 
tween the Sidereal and rest, this difference could not 



the Solar Day. have arigen . ^ g()lar day ig & 

result of the Earth's motion in its orbit, combined with 
its rotation. 

354. Moreover, the Earth's motion in its orbit is not 

Consequently, the 



uniform, as we shall hereafter see. 



With Fig. 77, explain the difference between the sidereal and the solar day. By 
what is this difference caused ? 354. Is the solar day always of the same length ? 



196 CELESTIAL LATITUDE AND LONGITUDE. 

apparent motion of the Sun is not uniform, and solar days 
are not of the same length ; for it is evident that if the 
Earth sometimes travels faster, and therefore farther, in 
the interval of one rotation than it does in another, the 
observer a has farther to travel before he gets to c / and 
as the Earth's rotary motion is uniform, he requires more 
time. In a subsequent chapter it will be shown how this 
irregularity in the apparent motion of the Sun is obviated. 

355. Celestial Latitude and Longitude. — The apparent 
yearly motion of the Sun is so important that astronomers 
map out the celestial sphere by a second method, in order 
to indicate his motion more easily ; for as the plane of the 
celestial equator, like that of the terrestrial equator, does 
not coincide with the plane of the ecliptic, the Sun's dis- 
tance from the celestial equator varies every minute. 

To get over this difficulty, they make of the plane of 
the ecliptic a sort of second celestial equator. They apply 
the term Celestial Latitude to angular distances from it to 
the Poles of the Heavens, which are 90° from it north and 
south. They apply the term Celestial Longitude to the 
angular distance — reckoned on the plane of the ecliptic — 
from the position occupied by the Sun at the vernal 
equinox, reckoning from right to left up to 360°. This 
latitude and longitude may be either heliocentric or 
geocentric, — that is, reckoned from the centre of either 
the Sun or the Earth. 

356. The Zodiac and its Signs. — The celestial equator 
in this second arrangement is represented by a circle 
called the Zodiac, which is divided, not only into degrees^ 
etc., like all other circles, but also into Signs of 30° each. 
These, with their symbols, are as follows : — 

Why not? 355. What have astronomers done, in order to indicate the Sun's 
motion more easily? What is meant hy Celestial Latitude and Celestial Lon- 
gitude ? What is the meaning of the terms heliocentric and geocentric, as applied 
to celestial latitude and longitude ? 356. In this arrangement, how is the celestial 
equator represented ? How is the Zodiac divided? Name the spring signs ; the 
summer signs : the autumn signs : the winter signs. With what must these 



SIGNS OF THE ZODIAC. 197 

Spring Signs. Summer Signs. Autumn Signs. Winter Signs. 

T Aries. 55 Cancer. ^= Libra. \3 Capricorn, 

tf Taurus. Q Leo. M Scorpio. /& Aquarius. 

n Gemini. ^l Yirgo. $ Sagittarius. 7$. Pisces. 

At the time this division was adopted, the Sun entered 
the constellation Aries at the vernal equinox, and traversed 
in succession the constellations bearing the above names ; 
but at present, owing to the Precession of the Equinoxes, 
which will be explained hereafter, the signs no longer 
correspond with the constellations, and must not therefore 
be confounded with them. 

357. Relation between the Ecliptic and the Celestial 
Equator. — These two methods of dividing the celestial 
sphere, and of determining the places of the heavenly 
bodies in it, refer, one to the plane of the terrestrial 
equator, and the other to the plane of the ecliptic. Now 
two things must be remembered : — (1) The angle formed 
by the celestial equator with the plane of the ecliptic is 
the same as that formed by the terrestrial equator, — that is, 
23^-° nearly. (2) The poles of the heavens are the same 
distance (23|°) from the celestial poles. 

Moreover, if we regard the centre of the celestial 
sphere as lying at the centre of the Earth, it is clear that 
the two planes will intersect each other at that point; 
half of the ecliptic will be north of the celestial equator, 
and half below it ; and there will be two points opposite 
to each other at which the ecliptic will cross the celestial 
equator. 

358. Apparent Path of the Sun. — As the Sun keeps to 
the ecliptic, it must, at different parts of its path, cross 
the celestial equator, be north of it, cross it again, and be 
south of it ; in other words, its latitude remaining the 

signs not be confounded ? Why were the names of the constellations given to 
them, if the signs and constellations do not agree? 357. What must be remem- 
bered with respect to the celestial equator and the poles of the heavens ? If the 
centre of the Earth be taken as the centre of the celestial sphere, what will 
follow? 358. Describe the apparent path of the Sun. Give an account of its 



198 APPABENT PATH OF THE SUN. 

same, its decimation or distance from the celestial equator 
will change. 

Hence, although the Sun rises and sets every day, its 
daily path is sometimes high, sometimes low. At the 
vernal equinox, when it occupies one of the points in 
which the ecliptic cuts the equator, it rises due east, and 
sets due west, like an equatorial star ; then, as it gradually 
increases its north declination, its daily path approaches 
the zenith, and its rising and setting points advance north- 
ward, until it reaches that part of the zodiac at which 
the planes of the ecliptic and equator are most widely 
separated. Here it appears to stand still; we have the 
summer solstice (from the Latin sol, the sun, and stare, to 
stand), and its daily path is similar to that of a star of 
23|-° north declination. It then descends through the 
autumnal equinox to the winter solstice, when its apparent 
path is similar to that of a star of 23-J° south declination, 
and its rising and setting points are low down toward the 
south. 

359. To determine the Time of Sunrise and Sunset 
with the Celestial Globe. — The use of the celestial globe 
throws light on many points connected with the Sun's 
apparent motion. When we have rectified the globe, as 
directed in Art. 347, its top will represent the zenith, — a 
miniature terrestrial globe, with its axis parallel to that of 
the celestial one, being supposed to occupy the centre of 
the latter. By bringing different parts of the ecliptic to 
the brass meridian, the varying meridian height of the Sun, 
on which the seasons depend, is at once shown. 

360. In addition to this, if we find from the almanac 
the position of the Sun in the ecliptic on any day, and 
bring it to the brass meridian, the globe shows the position 
of the Sun at noonday ; the index-hand is, therefore, set to 

movements after passing the vernal equinox. At the summer solstice. What 
does its apparent daily path resemble at the winter solstice ? 359. How may the 
varying meridian height of the Sun be shown with the celestial globe ? 360. How 



SUNRISE AND SUNSET— DAY AND NIGHT. 199 

12. If we then turn the globe westward till the Sun's 
place is brought close to the wooden horizon, we have 
sunset represented, and the index will indicate the time 
of sunset. If, on the other hand, we turn the globe east- 
ward from the brass meridian till the Sun's place is brought 
close to the eastern edge of the wooden horizon, we have 
sunrise represented, and the index indicates the time of 
sunrise. 

If the path of the Sun's place, when the globe is turned 
from the point occupied at sunrise to the point occupied 
at sunset, be carefully followed with reference to the hori- 
zon, the Diurnal Arc described by the Sun that day will 
be shown. 

361. To find the Length of Day and Night. — At noon 
and midnight the Sun is mid-way between the eastern and 
western points of the horizon — part of his path being 
above the horizon, and part below it. The time, there- 
fore, from noon to sunset is the same as from sunrise to 
noon. Similarly, the time from midnight to sunrise is 
equal to that from sunset to midnight. 

As civil time divides the twenty-four hours into two 
portions, reckoned from midnight and noon, we have a 
convenient way of finding the length of the day and night 
from the times of sunrise and sunset. For instance, if the 
Sun rises at 7, the time from midnight to sunrise is seven 
hours; but this time is equal, as has been seen, to the 
time from sunset to midnight ; therefore the night is four- 
teen hours long. Similarly, if the Sun sets at 8, the day is 
twice eight, or sixteen, hours long. Hence these rules : — 
Double the time of the Sun's setting is the length of 

the day. 
Double the time of the Sun's rising is the length of 
the night. 

may the time of sunrise and sunset be determined ? How may the Diurnal Arc 
described by the Sun be shown? 361. State two rules for finding the length of 
day and night from the time of sunrise and sunset. Give an example. Give the 



200 THE MOON'S PATH. 

Apparent Movements of the Moon. 

362. The Moon, we know, makes the circuit of the 
Earth in 29-| days ; in one day, therefore, supposing her 
motion to be uniform, she will travel eastward over the 
face of the sky a space of about 12° (360 ~- 29^ = 12^ 
nearly). According^, at a given hour, from night to 
night, her place will be changed about 12°, and she rises 
and sets later in consequence. 

Now, if the Moon's orbit were exactly in the plane of 
the ecliptic, we should not only have two eclipses every 
month (as heretofore stated), but she would appear always 
to follow the Sun's track. We have seen, however, that 
her orbit is inclined 5° to the plane of the ecliptic, and 
therefore to the Sun's apparent path. It follows, there- 
fore, that when the Moon is approaching her descending 
node, her path dips down (and her north latitude decreases), 
and that when she is approaching her ascending node, her 
path dips up (and her southern latitude decreases). 

The inclination of the Moon's orbit to the plane of the 
ecliptic being 5°, the greatest possible difference between 
her meridian altitudes is twice the sum of 5° and 23^-°, or 
57°. That is to say, she may be 5° north of a part of the 
ecliptic which is 23J° north of the equator, or she may be 
5° south of a part of the ecliptic which is 23-§° south of the 
equator. 

But let us suppose the Moon to move actually in the 
ecliptic. It is clear that the full Moon at midnight occu- 
pies exactly the opposite point in the ecliptic to that occu- 
pied by the Sun at noon-day. In winter, therefore, when 
the Sun is lowest, the Moon is highest ; and so in winter 
we get more moonlight than in summer, not only because 

reasoning on which these rules are "based. 362. Why does the Moon rise and 
set later from night to night? If the Moon's orbit were exactly in the plane of 
the ecliptic, what would follow ? How much is its orbit inclined to the plane of 
the ecliptic? What is the consequence? What is the greatest possible dif 
ference between the meridian altitudes of the Moon ? At what season do we 



THE HARVEST MOON. 



201 



the nights are longer, but because the Moon, like the Sun 
in summer, is best situated for lighting up the northern 
hemisphere. 

363. The Harvest Moon, — Although, as we have seen, 
the Moon advances about 12° in her orbit every 24 hours, 
the interval between two successive moonrises varies con- 
siderably. If the Moon moved along the celestial equator, 
the interval would always be about the same, because the 
equator is always inclined the same to our horizon. But 
she moves nearly along the ecliptic, which is inclined 23^° 
to the equator; and because it is so inclined, she ap- 
proaches the horizon at very different angles at different 
times, varying in lat. 40° between 21° and 79°. 

In Art. 357 we saw that half of the ecliptic is to the 
north and half to the south of the equator ; the ecliptic 
crosses the equator in the signs Aries and Libra. Now, 
when the Moon is farthest from these two points twice, a 
month, her path is parallel to the equator, and the interval 
between two risings will be nearly the same for two or 
three days together; but mark what happens if she be 
b: near a node, i. e., 

in Aries or Libra. 

In Aries the eclip- 
>-"' D tic crosses the equa- 



H" M W. _ 


ll/'-'"" 


"' 


_ T „ C 


tor to the north ; in 
Libra, to the south. 
In Fig. 78 the line 
H represents the 
horizon, looking 
east ; E Q the equa- 
tor, which in lat. 
40° is inclined 50° 


///W//////Mw/////^ 

: A 
Fig. 78.— Explanation of the Haevest Moon. 



get the most moonlight ? Explain how this happens. 363. What is said of the 
interval between two successive risings of the Moon ? Why does this interval 
vary? Under what circumstances is the time of the Moon's rising nearly the 
same for two or three successive days ? Explain this with Fig. 78. How often 



202 THE HAEVEST MOON. 

to the horizon. The dotted line A B represents the di- 
rection of the ecliptic when the sign Libra is on the hori- 
zon, and CD its direction when Aries is on the horizon. 

Now, the Moon appears to rise because our horizon is 
carried down toward it. It follows that, when the Moon 
occupies the three positions shown on the line 6 y 2>, she 
will rise nearly at the same time on successive evenings ; 
though she has advanced each time 12° in her orbit, she 
has got very little farther below the horizon, as will be 
seen in the figure. On the other hand, on the line A I>, 
her path being much more inclined to the horizon, each 
advance of 12° in her orbit carries her much farther below 
the horizon, and the difference between two successive 
risings will be greater in proportion. In lat. 40° this dif- 
ference may be no more than 25 minutes, and on the other 
hand may amount to an hour and a quarter. 

These successive risings of the Moon at nearly the 
same hour of course occur every month, as the Moon makes 
an entire circuit in a month and must pass the node in 
question in every circuit ; but they are not noticed, except 
when the Moon is full at this node in Aries, which can 
happen only within a fortnight of September 23 d. The 
full disk, seen above the horizon shedding its flood of light 
as soon as the Sun has set, seems to prolong the day, — 
most acceptably to the farmer, who at this time in Eng, 
land is busily engaged in gathering the fruits of the earth. 
Hence this is called the Harvest Moon. 

Apparent Movements of the Planets. 

364. The planets, when visible, appear as stars, and, 
like the stars, rise and set by virtue of the Earth's rota- 
tion. We need, therefore, to consider only their apparent 

do these successive risings at nearly the same hour occur ? When only are they 
noticed? What is the Moon called at this time? 364. How do the planets, when 
they are visible, appear? What apparent motions have they? Which of these 



APPAKENT MOVEMENTS OF THE PLANETS. 



203 



motions among the stars, caused by the Earth's revolution 
round the Sun, combined with their own actual movements. 
365." Distances of the Planets from the Earth. — As the 
planets revolve round the Sun in orbits of very different 
size and at different rates of speed, their distances from 
each other and from the Earth are perpetually varying. 

366. The Earth at one time has a given planet on the 
same side of the Sun as herself, and at another on the op- 
posite side. The extreme distances, therefore, between 
the Earth and a superior planet will vary by the diameter 
of the Earth's orbit — that is, in round numbers, by 183,- 
000,000 miles. In the case of an inferior planet, the ex- 
treme distances will differ by the diameter of the inferior 
planet's orbit. But this is not all ; as the orbits are ellip- 
tical and the nearest approaches and greatest departures oc- 
cur in different parts of them, the distance of any planet from 
the Earth even at these times will not always be the same. 

367. The following table shows the average least and 
greatest distance of each planet from the Earth, leaving 
out of account the variation due to the ellipticity of the 
orbits. The first column presents the difference between 
the distances of each planet and the Earth from the Sun, 
and the second column gives their sum. 





Least Distance. 


Greatest Distance. 




Miles. 


Miles. 


Mercury, 


. . 56,038,000 . 


126,823,000 


Yen us, . 


. . . 25,299,000 . 


157,562,000 


Mars, . 


. . 47,882,000 . 


230,742,000 


Jupiter, 


. . 384,263,000 . 


. 567,123,000 


Saturn, . 


. . 780,704,000 . 


. 963,565,000 


Uranus, 


. 1,662,421,000 . 


1,845,281,000 


Neptune, 


. 2,654,841,000 . 


2,837,701,000 



are we to consider ? 335. What is said of the distances of the planets from each 
other and from the Earth? 356. What difference must there be between the 
greatest and least distance of a superior planet from the Earth ? Of an inferior 
planet? What further affects the difference of distance? 367. What are shown 
in the tabic? How are the numbers in the first column obtained? How, those 



I 



204 APPAKENT MOVEMENTS OF THE PLANETS. 

368. To variations of distance are to be ascribed the 
striking changes of the planets in size and brilliancy at 
different times. The difference of size is greatest in the 
case of those planets whose orbits lie nearest that of the 
Earth, as shown in the table. Thus Venus, when nearest 
the Earth, appears six times larger than when it is farthest 
away, because it is really six times nearer to us ; while the 
apparent size of Uranus and Neptune is hardly affected, 
as the diameter of the Earth's orbit is small compared 
with their distance from the Sun. 

369. Phases of the Planets. — In the case of the planets 
which lie between us and the Sun, phases similar to those 
of the Moon are presented, because sometimes the planet 
is between us and the Sun, as is the case with the'Moon 
when it is new ; sometimes the Sun is between us and the 
planet, and consequently we see the illuminated hemi- 
sphere. At other times, as shown in Fig. 80, the Sun is to 
the right or left of the planet as seen from the Earth ; and 
a part of both the bright and the dark hemisphere is pre- 
sented to us. Among the superior planets, Mars is the 

' only one that exhibits a marked phase, which resembles 
that of the gibbous Moon. 

370. Aspects of the Planets. — By the Aspects of the 
planets are meant their positions in their orbits relatively 
to the Sun and the Earth. The aspects most frequently 
alluded to are Conjunction, Opposition, and Quadrature. 

When an inferior planet is in a line between the Earth 
and Sun, it is said to be in inferior conjunction with the 
Sun ; when it is in the same line, but beyond the Sun, it ^s 
said to be in superior conjunction. 

When a superior planet is on the opposite side of the 

in the second ? 368. What changes in the planets are to be ascribed to variations 
in their distance from the Earth? In what planets are these changes greatest? 
Compare Venus with Neptune in this respect. 369. What planets exhibit phases, 
and why? What superior planet exhibits a marked phase, and what does it 
resemble ? 370. What is meant by the Aspects of the planets ? What aspects 
are most frequently alluded to? When is a planet said to be in conjunction? 



ASPECTS OF THE PLANETS. 



205 



Sun, — that is, when the Sun is between us and it, — we say- 
it is in conjunction ; when in the same straight line, but 
with the Earth in the middle, we say it is in opposition, 
because it is then in the part of the heavens opposite to 
the Sun. 

When a planet is 90° from the position it occupies in 
conjunction and opposition, it is said to be in quadrature. 

Conjunction is denoted by the sign 6 ; opposition, by 
the sign 8 ; quadrature, by the sign □ . 

In Fig. 79, S represents the 
Sun, and E the Earth. V is Venus 
in inferior conjunction; W is the 
same planet in superior conjunction. 
Mars is shown in conjunction at 
A 7 ", in opposition at M } and in 
quadrature at Q. 

371. Transits. — The pas- 
sage of an inferior planet 
across the Sun's disk is 
called its Transit. In Fig. 
79, Venus at V is making 
her transit. 
A transit can take place 
only when a planet is in inferior conjunction. But, as the 
orbits of the planets do not lie in the plane of the ecliptic, 
there may be inferior conjunctions without any transit. 
Venus may be seen from the Earth in the same quarter as 
the Sun, and yet lie out of the plane which contains the 
centres of the Sun and the Earth. 

372. Elongations. — If an observer could watch the 
motions of the planets from the Sun, he would see them 
all pursuing their courses, always in the same direction, 
with different velocities, but in the case of any particular 




Fig. 79.— Conjunction— Opposition- 
Quadrature. 



When, in opposition? When, in quadrature? By what signs are these aspects 
denoted? Illustrate these positions with Fig. 79. 371. What is a Transit? 
When alone can a transit take place ? May there be inferior conjunctions with- 
out any transit ? 372. What complicates the movements of the planets, as seen 



206 ELONGATIONS OF THE PLANETS. 

one with an almost uniform rate of speed. Not only, how- 
ever, is our Earth a moving observatory, the motion of 
which complicates the apparent movements of the planets 
in an extraordinary degree, but from its position in the 
system all the planets are not seen with equal ease. 

In the first place, it is evident that only the superior 
planets are ever visible at midnight, as they alone can 
occupy the region of the heavens opposite to the Sun's 
place at that time, which is the region brought round to 
us at midnight by the Earth's rotation. Secondly, not 
only are the inferior planets always apparently near the 
Sun, but when they are nearest to us their dark sides are 
turned toward us, as they are then betAveen us and the 
Sun, and the Sun is shiniDg on the side turned away 
from us. 

The greatest angular distance, in fact, of Mercury and 
Venus from the Sun, either to the east (left) or west 
(right) of it, called the Eastern and the Western Elonga- 
tion, is, as heretofore stated, 29° and 47° respectively. 
Consequently, our only chance of seeing these planets is 
either in the day-time (generally with the aid of a good 
telescope), or just before sunrise at a western elongation, 
or after sunset at an eastern elongation. 

373. Stationary-points — Retrograde Motion. — In Fig. 
80 are shown the Earth in its orbit (P), and an inferior 
planet at its conjunctions and elongations. It is obvious 
that the rate and direction of the planet, as seen from the 
Earth, which for the sake of simplicity we will suppose to 
remain at rest, will both vary. At superior conjunction 
(SC) the planet will appear to move in the direction indi- 
cated by the outside arrow ; when it arrives at its eastern 

from the Earth? What planets alone are visible at midnight, and why? Where 
are the inferior planets always situated, and when are they invisible to us? 
What is meant by the Elongation of a planet ? What is the greatest elongation 
of Mercury ? Of Verms ? What are our only times for seeing these planets ? 
373. What does Fig. 80 represent ? With the aid of the figure, and supposing the 
Earth to be at rest, explain the apparent course of the inferior planet, its 



STATIONARY-POINTS, RETROGRADATIONS. 



207 



fSiM 



Fig. 80.— Elongations, Stationary-points, and Ketrogradations, of the 
Planets. 

elongation (EE), it will appear to be stationary, because 
it is then for a short time travelling exactly toward the 
Earth. From this point, instead of journeying from right 
to left, as at superior conjunction, it will appear to us to 
travel from left to right, or retrograde, until it reaches the 
point of westerly elongation ( WE), when for a short time 
it will travel exactly from the Earth, and again appear 
stationary, after which it recovers its direct motion. 

The only difference made by the Earth's own move- 
ments in this case is, that, as its motion is in the same 
direction as that of the inferior planet, the intervals be- 
tween two successive conjunctions or elongations will be 
longer than if the Earth were at rest. 

374. The superior planets, as seen from the Earth, 
appear to reach stationary-points in the same manner, but 
for a different reason. At the moment a superior planet 
appears stationary, the Earth, as seen from that planet, 
has reached its point of eastern or western elongation. 
Let JP in Fig. 80 represent a superior planet at rest, and 
let the inferior planet represented be the Earth. From 
the western elongation through superior conjunction, the 
motion of the planet referred to the stars beyond it will 
be direct — i. e. from *1 to *2, as shown by the outside 



stationary-points, and retro«rauation. What difference will be made by the 
Earth's movements? 374. Explain in the same way the apparent movements oi 



208 SYNODIC PEEIOD OF THE PLANETS. 

arrow. When the Earth is at its eastern elongation, as 
seen from the planet, the planet as seen from the Earth 
will appear at rest, as we are advancing for a short time 
straight to it. When this point is passed, the apparent 
motion of the planet will be reversed ; it will appear to 
retrograde from *2 to *1, as shown by the inside arrow. 

As in the former case, the only difference when we deal 
with the planet in motion, will be that the times in w T hich 
these changes take place will vary with the actual motion 
of the planet ; for instance, it will be much less in the case 
of Neptune than in that of Mars, as the former moves 
much more slowly. 

375. Synodic Period. — In consequence of the Earth's 
motion, the period in which a planet regains the same 
position with regard to the Earth and Sun is different 
from the actual period of the planet's revolution round the 
Sun. The time in which a position, such as conjunction 
or opposition, is regained, is called a Synodic Period. The 
synodic periods of the different planets are as follows : — 

Mean Solar Mean Solar 

Days. 

Mercury, . . 115.87 

Venus, . . 583.92 

Mars, . . . 779.94 

Jupiter, . . 398.87 
These synodic periods have been found by actual 
observation, and from them the times of the planets' revolu- 
tion round the Sun have been obtained. 

376. Inclinations and Nodes of the Orbits. — If the mo- 
tions of the planets were confined to the plane of the eclip- 
tic, they would, as seen from the Earth, resemble those 
of the Sun ; but their orbits are all more or less inclined 

a superior planet, supposed to be at rest. What difference will the motion of the 
planet make ? 375. What is meant by a Synodic Period ? Why does a planet's 
synodic period differ from its period of revolution round the Sun? State the 
synodic periods of the different planets. How have they been found? What 
have been obtained from them ? 376. What causes the apparent motion of the 



Saturn,, . . 378.09 
Uranus, . . 369.66 
Neptune, . . 367.49 



INCLINATION OF THE PLANETAKY ORBITS. 



209 



to that plane (Art. 145). Here is a table of the present 
inclinations, and positions of the ascending nodes : — 



Fig. 81. 






Inclination of 


Longitude of 




Orbit. 


Ascending Node. 




o / // 


O 1 


Mercury, 


.705 


. . 45 57 


Venus, . 


. 3 23 29 


. . 74 51 


Mars, . 


. 1 51 6 


. . 47 59 


Jupiter, 


. 1 18 52 


. . 98 25 


Saturn, . 


. 2 29 36 


. . Ill 56 


Uranus, . 


. 46 28 


. . 72 59 


Neptune, 


. 1 46 59 


. .130-6 



377. The apparent distance of a 
planet from the plane of the ecliptic 
will "be greater, as seen from the 
Earth, if the planet is nearer the 
Earth than the Sun at the time of 
observation. Hence, as the dis- 
tance of the planet from the Earth 
must be taken into account, the dis- 
tance above or below the plane of 
the ecliptic will not appear to vary- 
so regularly when seen from the 
Earth, as it would do could we ob- 
serve it from the Sun. 

Of course, when the planet is at 
a node, it will always appear in the 
ecliptic. 

378. Path of Venus among the 
Stars. — Fig. 81 represents the path 
of Venus, as seen from the Earth 
from April to October, 1868. A 

study of it should make what has been said about the 

planets to differ from that of the Sun? 377. Why does not the distance of a 
planet from the plane of the ecliptic vary as regularly, when seen from the Earth, 
as it would do if seen from the Sun ? When will a planet appear in the ecliptic ? 



210 



APPAKENT PATHS OF THE PLANETS. 



apparent motions of the planets quite clear. From April 
to June the planet's north latitude is increasing, while 
the node and stationary-point — which in this case coin- 
cide, though they do not always do so — are reached about 
the 25th of June. The southern latitude rapidly increases, 
until, on the 9th of August, the other stationary-point is 
reached, after which the south latitude decreases. 

379. Effect of the Ellipticity and Inclination of the 
Orbit in the case of Mars. — The apparent path of a 
planet, then, is affected by the motions of the Earth 




Fig. 82.— The Orbits of Maes and the Earth. 



378. What does Fig. 81 represent? Describe the path of Venus, as thus ex- 



OBBITS OF THE EAETH AND MAES. 211 

and the inclination of its own orbit. If we examine 
into the position of the orbit of Mars, for instance, more 
closely than we have hitherto done, we shall see how 
the ellipticity of the orbit and its inclination affect our ob- 
servations of the* physical features of the planet. Fig. 82 
shows the exact positions in space of the orbits of the 
Earth and Mars, and the amount and direction of the in- 
clination of their axes, and the line of the nodes of Mars ; 
both planets are represented in the positions they occupy 
at the winter solstice of the northern hemisphere. The 
lines joining the two orbits indicate the positions occupied 
by both planets at successive oppositions of Mars, at which 
times, of course, Mars, the Earth, and the Sun, are in the 
same straight line (leaving the inclination of the orbit of 
Mars out of the question). 

It is seen that at the oppositions of 1830 and 1860 the 
two planets were much nearer together than in 186V or 
1869. 

380. Fig. 82 also enables us to understand that, in the 
case of an inferior planet, if we suppose the perihelion of 
the Earth to coincide in direction with (or, as astronomers 
put it, to be in the same heliocentric longitude as) the 
aphelion of the planet, the conjunctions which happen in 
this part of the orbits of both will bring the bodies nearer 
together than will the conjunctions which happen else- 
where. Similarly, if we suppose the aphelion of the Earth 
to coincide with the perihelion of a superior planet, as in 
the case of Mars, the opposition which happens in that 
part of the orbit will be the most favorable for observa- 
tion. The Earth's orbit, however, is practically so nearly 
circular that the variation depends more upon the eccen- 
tricity of the orbits of the other planets than upon our 
own. 



hibited. 379. By what is the apparent, path of a planet affected? What does 
Fie. 82 represent ? What is seen with respect to the nearness of the planets at 
different oppositions ? 380. What may also be understood from Fig. 82? What 



212 



APPEARANCES OF SATUEN'S EINGS. 



381. Fig. 82 also shows us that, when Mars is observed 
at the solstice indicated, we see the southern hemisphere 
of the planet better than the northern one ; while at those 
oppositions which occur when the planet is at the opposite 
solstice, the northern hemisphere is chiefly visible. But 
we see more of the northern hemisphere in the latter case 
than we do of the southern one in the former, because in 
the latter case the planet is above the ecliptic, and we 
therefore see under it better ; in the former it is below the 
ecliptic, and we see less of the southern hemisphere than 
we should do were the planet situated in the ecliptic. 




Fig. 83.— Different Appearances of Saturn's Eings. 

382. Saturn's Rings as seen at Different Times from 
the Earth. — Fig. 83 shows the effect of inclination in 
the case of the rings of Saturn. The plane of the rings is 
inclined to the ecliptic, and the different positions of this 



opposition will be the most favorable for observation ? 381. When is the southern 
hemisphere of Mars best seen, and when the northern ? Why at the latter time 
do we see the northern hemisphere more fully than we see the southern hemi- 
sphere at the former ? 382. What is shown in Fig. 83 ? What different appear- 



THE EINGS SOMETIMES INVISIBLE. 



213 




Fig. 84.— Appearance op Saturn 
¥een the plane of its klngs 
passes through the earth. 



plane are always parallel. 
Twice in the planet's year 
the plane of the rings must 
pass through the Sun; and 
while the plane is sweeping 
across the Earth's orbit, the 
Earth, in consequence of its 
rapid motion, may pass two 
or three times through the 
plane of the ring. 
Hence the ring-system about this time may be invisi- 
ble, from three causes : (1) Its plane may pass through 

the Sun, and its extremely thin edge only will be lit up ; 

(2) The plane may pass through the Earth ; or (3) The 

Sun may be 

lighting up one 

surface, and 

the other may 

be presented 

to the Earth. 

These changes 

occur about 

every fifteen 




years 



s, and in 



Fig. 85.— Saturn with the North Surface 
Kings presented to the Earth. 



the mid-inter- 
val the surface of the rings — sometimes the northern one, 
at others the southern — is presented to the Earth in the 
greatest angle. 

In Fig. 63, page 148, Saturn was shown with the 
south surface of its ring-system presented to view. In 
Fig. 85, we have the aspect of the planet when the north 
surface of the ring is visible. 



ances are presented by the rings ? What three causes may render the ring- 
system invisible ? How often do these changes occur ? How is the ring-system 
presented to the Earth in the mid-interval ? How is Saturn represented in Fig. 
63 ? How, in Fig. 85 ? 



214 THE MEASUREMENT OF TIME. 

CHAPTER XIII 

THE MEASUREMENT OF TIME. 

383. Having dealt with the apparent motions of the 
heavenly bodies, we now come to what those apparent 
motions accomplish for us, — namely, the division and exact 
measurement of time. For common purposes, time is 
measured by the Sun, as it is that body which gives us .the 
primary division of time into day and night ; but for as- 
tronomical purposes the stars are used, as the apparent 
motion of the Sun is subject to variation. 

The correct measurement of time is not only one of 
the most important parts of practical Astronomy, but it is 
one of the most direct benefits conferred on mankind by 
the science ; it enters, in fact, so much into every affair of 
life, that we are apt to forget that there was a period 
when that measurement was all but impossible. 

384. Clepsydrae and Sun-dials. — Among the contriv- 
ances which were to the ancients what clocks and watches 
are to us, we may mention Clep'sydrae, or water-clocks, 
and Sun-dials. Of these, the former seem to have been 
the more ancient, and were used not only by the Greeks 
and Romans, but by other nations, the ancient Britons 
among them. In its simplest form it resembled the hour- 
glass, water being used instead of sand, and the flow of 
time being measured by the flow of the water. 

After the time of Archimedes, clepsydrae of the most 
elaborate construction were common ; but while they were 
in use, the days, both winter and summer, were divided 
into twelve hours from sunrise to sunset, and consequently 
the hours in winter were shorter than the hours in sum- 



383. What does Chapter XIII. treat of? How is time measured for common 
purposes ? How, for astronomical purposes ? What is said of the importance 
of the correct measurement of time? 384 What instruments did the ancients 
use for measuring time ? By whom were clepsydrae employed ? Describe the 



THE SUN-DIAL. 215 

mer. The clepsydra, therefore, was almost useless except 
for measuring intervals of time, unless different ones were 
employed at different seasons of the year. 

385. The sun-dial, also, is of great antiquity; it is re^ 
ferred to as in use among the Jews 742 b. c. This was a 
great improvement on the clepsydra ; but at night and in 
cloudy weather it could not be used, and the rising, cul- 
mination, and setting of the various constellations, were 
the only means available for approximately telling the 
time during the night. Euripides, who lived 480-407 b. c, 
makes the Chorus in one of his tragedies ask the time in 
this form : — 

" What is the star now passing ? " 

and the answer is, 

" The Pleiades show themselves in the east; 
The Eagle soars in the summit of heaven." 

It is on record that as late as a. d. 1108 the sacristan of the Abbey 
of Cluny consulted the stars when he wished to know whether the time 
had arrived to summon the monks to their midnight prayers ; in other 
cases, a monk remained awake, and to measure the lapse of time repeated 
certain psalms, experience having taught him in the day, by the aid of 
the sun-dial, how many psalms could be said in an hour. 

To tell the passing hours, Alfred the Great (985 a. d.) used wax 
candles twelve inches in length. Marks on the surface at equal intervals 
denoted hours and their subdivisions, each inch of candle burnt showing 
that about twenty minutes had passed. To prevent currents of air from 
making his candles burn irregularly, he enclosed them in cases of thin 
transparent horn. 

386. Construction of the Sun-dial. — To understand the 
construction of the sun-dial, let us imagine a transparent 
cylinder, having an opaque axis, both axis and cylinder 
being placed parallel to the axis of the Earth. If the 

clepsydra in its simplest form. When did clepsydrae of elaborate construction 
become common ? What difficulty interfered with their usefulness? 385. How 
early is the sun-dial known to have been in use? How did it compare with the 
clepsydra? How was the hour told at nh?ht? What question and answer occur 
in one of the tragedies of Euripides ? How was the time for summoning the 
monks to their midnight prayers determined, as late as 1108 a. d. ? To what 



216 



MEASUKEMENT OF TIME. 



cylinder be exposed to the Sun, the shadow of the axis 
will be thrown on the side of the cylinder away from the 
Sun ; and, as the Sun appears to travel round the Earth's 
axis in 24 hours, it will also appear to travel round the 
axis of the 
cylinder in the 
same time, and 
will cast the 
shadow of the 
axis on the side 
of the cylinder 
as long as it re- 
mains above 
the horizon. 

All we have 
to do, therefore, 
is to trace on 
the side of the 
cylinder 24 
lines 15° apart 
(360-^-24 = 15), 
taking care to 
have one line 
due north of the axis. When the Sun is south, at noon, 
the shadow of the axis will be thrown on this line, which 
we mark XII. When the Sun has advanced 15° to the 
west, the shadow will be thrown on the next line to the 
east, which we mark I., and so on. The distance of the 
Sun above the equator will evidently make no difference 
in the lateral direction of the shadow. 

387. In practice, however, we do not need the cylinder; 
all we want is a projection called a Style, parallel to the 
Earth's axis, and a Dial. The dial may be upright, hori- 
zontal, or inclined in any way so as to receive the shadow 




Fig. 86.— Sun-dial. A B, axis of cylinder ; J/2V, P Q, two 
dials, at different angles to the plane of the horizon, 
showing how the imaginary cylinder determines the 
hour-lines. 



device for measuring time did King Alfred resort ? 380. Explain the construc- 
tion of the sun-dial. 387. Do we really need a cylinder? What are needed? 



CLOCKS AND WATCHES. 217 

of the style; the lines on it indicating the hours will 
always be determined by imagining such a cylinder as is 
described above, cutting it parallel to the plane of the 
dial, and then joining the hour-lines on its surface with 
the style where it meets the dial. 

388. Clocks and Watches. — The principle of both clocks 
and watches is that a number of wheels and pinions, work- 
ing one in another, are forced to turn round, and are pre- 
vented from doing so too quickly. The force which gives 
the motion may be either a weight or a spring : the force 
which regulates the motion may proceed either from a 
pendulum, which at every swing locks the wheels, or from 
some equivalent arrangement. 

389. Clocks appear to have been first used in Europe 
in the monasteries in the eleventh century ; their invention 
is attributed to the Saracens. The first clock made in 
England, 1288 a. d., was considered so great a work that 
a high dignitary was appointed to take care of it, and paid 
for so doing from the public treasury. 

Tycho Brahe used a clock, the motion of which was 
regulated by means of an alternating balance formed by 
suspending two weights on a horizontal bar, the move- 
ment being made faster or slower by altering the distances 
of the weights from the middle of the bar. But the clock, 
as an accurate measurer of time, dates from the middle of 
the seventeenth century, when the pendulum was intro- 
duced as a regulator by Galileo and Huyghens. 

390. The Mean Sun, — In both clocks and watches we 
mark the flow of time by seconds, sixty of which make a 
minute, sixty minutes making an hour, and twenty-four 
hours a day. To the astronomer, however, the meaning 

How may the dial be placed? 388. Oil what principle are both clocks and 
watches constructed? What is the force that imparts the motion? How is the 
motion regulated ? 389. When were clocks first used in Europe ? To whom is 
their invention attributed ? When was the first clock made in England ? How 
was it regarded? How did Tycho Brahe regulate his clock? When and by 
whom was the pendulum introduced as a regulator ? 390. What does the word 

10 



218 MEASUEEMENT OF TIME. 

of the word day is indefinite, unless it is specified whether 
a solar or sidereal day is intended. As commonly used, 
the term means neither ; for when it was found that, in 
consequence of the irregularity of the Earth's motion in 
its orbit, the solar days differ in length, with the view of 
establishing a uniform measure of time for civil purposes, 
a civil day was made the average of all the solar days in 
the year. Our common day, therefore, is not measured by 
the true Sun, as a sun-dial measures it, but by what is 
called the mean Sun. 

391. Irregularities of the Sun's Apparent Daily Motion. 
— Let us inquire into the motion of the imaginary mean 
Sun, by means of which the irregularities of the Sun's 
apparent daily motion are obviated. 

In the first place, the real Sun's motion is in the ecliptic, 
and is variable. Secondly, the Sun crosses the equator 
twice a year at the equinoxes, at an angle of 23^°, while 
midway between the equinoxes its path is almost parallel 
to the equator. Hence, its real motion being performed 
at different angles to the equator, its apparent motion will 
vary when referred to that line, being least rapid when 
the angle is greatest. 

392. Let us first deal with the first cause — the in- 
equality of the real Sun's motion. When the Earth is 
nearest the Sun, about Jan. 1st, the Sun appears to travel 
through 1° 1' 10" of the ecliptic in 24 hours ; at aphelion, 
about July 1, the daily arc is reduced to 51' 12/ The 
first thing to be done, therefore, is to give a constant 
motion to the mean Sun. 

The real Sun passes through the entire circle, or 360°, 
in 365d. 5h. 48m. 46s., or about 365.2422 days. Hence the 
mean distance traversed in one day will be as many de- 



day, as commonly used, mean? By what is it measured? 391. Where does the 
real Sun move ? Is its motion uniform or variable ? What, besides, causes its 
motion to appear irregular ? 392. What is the length of the daily arc traversed 
by the Sun at perihelion and at aphelion ? Find the arc which the mean Sun 



THE TRUE SUN AND THE MEAN SUN. 219 

grees as 365.2422 is contained times in 360, — or a little 
more than 0.985 degrees. This distance, therefore, which 
equals 59' 8.33", is the arc which the mean Sun travels 
daily. 

393. If the true Sun moved in the equator instead of in 
the ecliptic, a table showing how far the mean and true Sun 
are apart for every day in the year would at once enable 
us to determine mean time. But the true Sun moves along 
the ecliptic, while the mean Sun must be supposed to move 
along the equator, so that it may be carried evenly round 
by the Earth's rotation. This brings out the second cause 
of the inequality of the solar days. 

At the solstices the true Sun moves almost parallel to 
the equator ; at the equinoxes it crosses the equator at an 
angle of 23-J- , and, when its motion is referred to the equa- 
tor, time is lost. This will be rendered evident if on a 
celestial globe we place wafers, equally distant from the 
first point of Aries, on both the equator and the ecliptic, 
and bring them to the brass meridian. 

We have also the mean Sun, not supposed to move 
along the ecliptic at all, but along the equator, at the uni- 
form rate of 59' 8.33" a day, and starting, so to speak, from 
the first point of Aries, where the ecliptic and equator in- 
tersect. Supposing the true Sun to move along the ecliptic 
at a uniform rate, its position referred to the equator would 
correspond with that of the mean Sun at the two solstices 
and the two equinoxes. 

But the motion of the true Sun is not uniform ; it moves 
fastest when the Earth is in perihelion, slowest when the 
Earth is in aphelion ; and, if we take this fact into account, 
we find that the real Sun and the meah Sun coincide in 



travels daily. 393. How does the true Sun move at the solstices ? What does it do 
at the equinoxes? Does it appear to move more slowly or rapidly at the latter 
points ? If the true Sun moved along the ecliptic at a uniform rate, at what points 
would it correspond with the mean Sun ? Taking the irregular motion of the true 
Sun into account, at what times do the real Sun and the mean Sun coincide 



I 



220 MEASUREMENT OF TIME. 

position four times a year; namely, at April 15th, June 
15th, August 31st, and December 24th. 

394. Equation of Time.— At the following dates the 
difference between apparent and mean time is as specified 
below : — 





Minutes. 




Minutes. 


February 11th, 


. + iH 


July 25th, . 


. + 6 


May 14th, . 


. — 4 


November 1st, 


. - 16i 



This is what is called the Equation of Time, and is what 
we must add to, or subtract from, the time shown by a 
sun-dial, to make it correspond with that of a correct clock. 
The sign + before the equation of time denotes that it is 
to be added; the sign — , that it is to be subtracted. 

When the Earth is in perihelion (or the Sun in perigee), 
the real Sun, moving at its fastest rate, gains on the mean 
Sun, and therefore takes longer than the mean Sun to come 
to the meridian ; hence the dial is behind the clock, and 
we must add the equation of time to the apparent time to 
get the mean time. When the Earth is in aphelion (or the 
Sun in apogee), the reverse holds good. In November, as 
shown by the above table, the true Sun sets 16m. earlier 
than it would do if it occupied the position of the mean 
Sun, by which our clocks are regulated. In February it 
sets 15m. later : hence at the beginning of the year the 
day lengthens more rapidly than it would otherwise do. 
We cannot obtain mean time at once from observation ; 
but, from an observation of the true Sun, by adding or sub- 
tracting the equation of time, as the case may be, it can be 
readily deduced. Mean time is now universally used in all 
civilized countries. 

395. Commencement of the Different Days. — We must 
next consider when the different days begin. We have, 

in position? 394. What is meant by the Equation of Time? What do the 
signs + and — mean, when prefixed to the equation of time ? When is the 
equation of time to be added, and when subtracted? What is the equation of 
time for Feb. 11th? For May 14th? For July 25th? For Nov. 1st? How is 



THE SOLAR AND THE CIVIL DAY. 221 

I. The Apparent Solar Day, reckoned from the in- 
stant the true Sun crosses the meridian, till it crosses 
it again. 

II. The Mean Solar Day, reckoned from the instant the 
mean Sun crosses the meridian, in the same manner. Both 
these days are used by astronomers. 

III. The Civil Day, commencing at midnight, and 
reckoned through 12 mean hours only to noon, and thence 
through another 12 hours to the next midnight. The civil 
reckoning is therefore always 12 hours in advance of the 
astronomical reckoning ; hence this rule for determining 
the latter from the former : — For p. m. civil times, make no 
change ; but for a. m. diminish the day of the month by 1 
and add twelve to the hours. Thus : Jan. 2d, 7h. 49m. p. m. 
civil time, is Jan. 2d, Vh. 49m. astronomical time ; but Jan. 
2d, 7h. 49m. a.m. civil time is Jan. 1st, 19h. 49m. astronomi- 
cal time. 

396. Length of the Different Days. — Expressed in mean 
time, the length of the day is as follows : — 

Apparent solar day, . . variable. 

Mean solar day, .... 24h. Om. Os. 

Sidereal day, 23 56 4.09 

Mean lunar day, . . . 24 54 

397. Sidereal Time is reckoned from the first point of 
Aries. When the mean Sun occupies this point, which it 
does at the vernal equinox, the mean-time clock and the 
sidereal clock will agree. But this happens at no other 
time, as the sidereal day is only 23h. 56m. 4s. (mean time) 
long ; so that the sidereal clock gains about four minutes a 
day, or one day a year, as compared with mean time. Of 

mean time obtained? 395. When does the apparent solar day begin? The 
mean solar day ? When does the civil day begin, and how is it reckoned ? How 
does the civil reckoning compare with the astronomical reckoning. Give the 
rule for changing civil time to astronomical time. Give an example. 396. What 
is the length of the apparent solar day ? Of the mean solar day ? Of the sidereal 
day, in mean time ? Of the mean lunar day ? 397. From what is sidereal time 
reckoned ? When will the mean-time clock and the sidereal clock airree ? When 



, 



222 MEASUREMENT OF TIME. 

course the coincidence is established again at the next ver- 
nal equinox. 

A sidereal clock represents the rotation of the Earth on 
its axis, as referred to the stars, its hour-hand performing 
a complete revolution through the 24 sidereal hours be- 
tween the departure of any meridian from a star and its 
next return to it. At the moment that the vernal equinox, 
or a star whose right ascension is Oh. Om. Os. is on the 
meridian of Greenwich, the sidereal clock ought to show 
Oh. Om. Os. ; and at the succeeding return of the star, or 
the equinox, to the same meridian, the clock ought to in- 
dicate the same time. 

398. The Week. — Although the week, unlike the day, 
month, and year, is not connected with the movements of any 
heavenly body, the names of the seven days of which it is 
composed were derived by the Egyptians from the seven 
celestial bodies then known. The Romans, in their names 
for the days, observed the same order, distinguishing them 
as follows : — 

Dies /Saturni, . . Saturn's day, . . Saturday. 

Dies Solis, . . . Sun's day, . . . Sunday. 

Dies Dunce, . . Moon's day, . . Monday. 

Dies Martis, . . Mars' day, . . . Tuesday. 

Dies Mercurii, . Mercury's day, . Wednesday. 

Dies Jovis, . . Jupiter's day, . . Thursday. 

Dies Veneris, . . Yenus's day, . . Friday. 

We see at once the origin of our English names for the 
first three days ; the remaining four are named from Tiw, 
Woden, Thor, and Frigga, northern deities equivalent to 
Mars, Mercury, Jupiter, and Yenus, in the classical my- 
thology. 



will they next agree ? Why do they not agree meanwhile ? What does a sidereal 
clock represent? 398. From what did the Egyptians name the days of the week? 
Who observed the same order in their names for the days? Give the Latin 
names for the flays of the week. Whence are their English names derived? 



THE MONTH. 223 

399. The Month.. — We next come to the month, a period 
regulated entirely by the Moon's motion round the Earth. 

The lunar month is the same as the lunation or synodic 
month, and is the time which elapses between two con- 
secutive new or full Moons, or in which the Moon returns 
to the same position relatively to the Earth and Sun. 

The tropical month is the revolution of the Moon with 
respect to the movable equinox. 

The sidereal month is the interval between two succes- 
sive conjunctions of the Moon with the same fixed star. 

The anomalistic month is the time in which the Moon 
returns to the same point (for example, the perigee or 
apogee) of her movable elliptic orbit. 

The nodical month is the time in which the Moon ac- 
complishes a revolution with respect to her nodes, the line 
of which is also movable. 

The calendar month is the month recognized in the al- 
manacs, and consists of different numbers of days, such 
as January, February, etc. 

400. Length of the Lunar and other Months. — The 
length of these different months is as follows : — 

Mean Time, 
d. h. m. s. 

Lunar, or Synodic month, . ... 29 12 44 2.84 

Tropical month, 27 7 43 4.71 

Sidereal month, 27 7 43 11.54 

Anomalistic month, 27 13 18 37.40 

Nodical month, 27 5 5 35.60 

401. The Year. — The year is the time of the Earth's 
revolution round the Sun, as the day is the period of its 
rotation on its axis. There are various sorts of years, as 
there are different kinds of days. Thus, we may take the 

399. By what is the month regulated ? What is the Lnnar Month ? The Tropical 
Month ? The Sidereal Month ? The Anomalistic Month ? The Nodical Month ? 
The Calendar Month? 400. Which is the longest of these different kinds of 
months ? Which is the shortest ? 401. What is the Year ? What is the Sidereal 
Year? The Solar, or Tropical, Year? The Anomalistic Year? Which is the 



224 MEASUREMENT OF TIME. 

time that elapses between two successive conjunctions of 
the Sun, as seen from the Earth, with a fixed star. This 
is called the Sidereal Year. 

Or we may take the period that elapses between two 
successive passages through the vernal equinox. This is 
called the Solar, or Tropical Year, and it is shorter than 
the sidereal year, in consequence of the precession of the 
equinoxes. The vernal equinox in its recession meets the 
Sun, which therefore passes through it sooner than it would 
otherwise do. 

Again, we may take the time that elapses between two 
successive passages of the Earth through perihelion or 
aphelion. As these points have a forward motion in the 
heavens, the Anomalistic Year, as this period is called, is 
longer than the sidereal year. 

402. Length of the Sidereal and other Years. — The 
exact length of these years is as follows : — 

Mean Time, 
d. h. m. s. 

Mean sidereal year, . . . . 365 6 9 9.6 
Mean solar or tropical year, . 365 5 48 46.05444 
Mean anomalistic year, . . . 365 6 13 49.3 

403. The Calendar. — It is seen from this table that the 
solar year does not contain an exact number of solar days, 
but nearly a quarter of a day over. It is said that the in- 
habitants of ancient Thebes were the first to discover this. 
The calendar had got in such a state of confusion in the 
time of Julius Caesar, that he called in the aid of the 
Egyptian astronomer, Sosigenes, to reform it. The latter 
recommended that one day every four years should be 
added, by reckoning the sixth day before the kalends of 
March (Feb. 24th) twice ; hence the term Bissextile (from 
the Latin Ms, twice, and sextus, sixth). 

longer, the solar or the sidereal year? The anomalistic or the sidereal year? 
402. What is the exact length of the mean solar year? 403. What caused the 
calendar to get in confusion in old times ? Who attempted to reform it ? Whom 



KEFOKMATION OF THE CALENDAR. 225 

Now, this arrangement was a great improvement ; but 
too much was added, and the matter was again looked into 
in the sixteenth century, by which time the over-correction 
had amounted to more than ten days, the vernal equinox 
falling on March 11th, instead of March 21st. Pope 
Gregory XIII., therefore, undertook to continue the good 
work begun by Julius Caesar, and made the following 
rule for the future : — Every year divisible by 4 (except the 
secular years, 1800, 1900, etc.) to be a bissextile, or leap- 
year, containing 366 days ; every year not so divisible to 
consist of only 365 days; every secular year divisible by 
400 to be a leap-year ; every secular year not so divisible 
to consist of 365 days. According to this arrangement, 
the error amounts to only 1 day in 3,866 years. 

404. Old and New Style. — The Julian Calendar (Julius 
Caesar's) was introduced 46 b. c. ; the Gregorian (Pope 
Gregory's), in 1582 a. d. The latter was not adopted in 
England till 1752, when the correction was made by drop- 
ping eleven days in September, the day following the 2d 
of that month being called the 14th. This was known as 
the New Style (1ST. S.), in contradistinction to the Old 
Style (O. S.). In Russia the old style is still retained, al- 
though it is customary to give both dates, thus : 1870 g^-^f ' 

405. It is all-important that the calendar be exactly 
adjusted to the length of the solar year; otherwise the 
seasons would not commence on the same day of the same 
month as they do now, but would in the course of time 
make the circuit of all the days in the year. January, or 
any other month, would fall successively in spring, sum- 
mer, autumn, and winter. 



did Caesar call to his aid? What improvement was made by Sosigenes ? What 
difficulty still remained ? By whom was this obviated ? What change was made 
by the Gregorian Calendar ? According to this calendar, what is the amount of 
error ? 404. When was the Julian Calendar introduced ? When, the Gregorian ? 
When was the Gregorian Calendar adopted in England ? How was the correctiou 
made? How were the two modes of reckoning distinguished? Where is the 
Old Style still retained? 405. Why is it important that the calendar should be 



226 ASTEONOMICAL INSTRUMENTS.. 

406. Change in the Length of the Solar Year. — At pres- 
ent, owing to a change of form in the Earth's orbit, the 
solar year is diminishing at the rate of T 6 ¥ of a second in a 
century. It is shorter now than it was in the time of 
Hipparchus by about 12 seconds. 

407. Change of Aphelion and Perihelion. — If the solar 
and the anomalistic year were of equal length, it would fol- 
low that, as the seasons are regulated by the former, they 
would always occur in the same part of the Earth's orbit. 
As it is, however, the line joining the aphelion and peri- 
helion points, termed the Line of Apsides, slowly changes 
its direction, at such a rate that in a period of 21,000 years 
it makes a complete revolution. At present, as already 
stated, we are nearest to the Sun about Jan. 1st; in a. d. 
6485, the perihelion will correspond with the vernal equi- 
nox. 



CHAPTER XIV. 
ASTRONOMICAL INSTRUMENTS. 

Light 

408. What Light is. — Modern science teaches us that 
Light consists of undulations or waves of a medium called 
ether, which pervades all space. These undulations are to 
the eye what sound-waves are to the ear, and they are set 
in motion by bodies at a high temperature — the Sun, for 
instance— much in the same manner as the air is put in 
motion by our voice, or the surface of water by throwing 
in a stone. But though a wave-motion results from all 



exactly adjusted to the length of the solar year? 406. At what rate is the solar 
year constantly changing, and why 407. What is meant by the Line of Apsides ? 
What change is this line undergoing? When are we at present nearest the Sun ? 
When will the perihelion correspond with the vernal equinox? 

408. Of what does Light consist? To what are waves of light analogous? 



LIGHT. 



227 



these causes, the way in which the wave travels varies in 
each case. 

409. Velocity of Light. — Though light moves so quickly 
that to us its passage seems instantaneous, it requires 
time to travel from an illuminating to an illuminated 
body. Its velocity was determined by Roemer, a Danish 
astronomer, from observations on the moons of Jupiter. 
He found that the eclipses of these moons (which he had 
calculated beforehand) happened 16m. 26s. later when 
Jupiter was in conjunction with the Sun than when he 
was in opposition. Knowing that Jupiter is farther from 
us in the former case than in the latter, by exactly the 
diameter of the Earth's orbit, he soon convinced himself 
that the difference of time was due to the fact that the 
light had so much farther to travel. Now, the additional 
distance, i. e., the diameter of the Earth's orbit, being 
183,000,000 miles, it follows that light travels about 185,000 
miles a second. This fact has been abundantly proved 

since Roemer's time, and 

\g what astronomers call the 

T aberration of light is one of 

the proofs. 

410. Aberration of Light. — 
We may get an idea of the 
aberration of light by observ- 
ing the way in which, when 
caught in a shower, we hold 
the umbrella inclined in the 
direction in which we are 
hastening, instead of overhead, 

Fig. 87.— Illustrating the Aber- as we should do were we Stand- 
ratiok of Light. ing ^ Let ug make ^ & 




409. By whom was the velocity of light determined? How was it determined ? 
What is the velocity of light ? What is one proof of the velocity thus established ? 

410. How may we get an idea of the aberration of light ? Illustrate it with Fig. 
87. To see a star, what must we do with our telescopes ? On what does the 



228 ASTKONOMICAL INSTRUMENTS. 

little clearer. Suppose we wish to let a drop of water 
fall through a tube (see Fig. 87) without wetting the 
sides. If the tube is at rest, there is no difficulty — it 
has only to be held upright in the direction A B ; but if 
we must move the tube, the matter is not so easy. The 
diagram shows that the tube must be inclined, or else the 
drop in the centre of the tube at a will no longer be in the 
centre at b ; and the faster the tube is moved, the more it 
must be inclined. 

Now, we may liken the drop to rays of light, and the 
tube to the telescope, and we find that to see a star we 
must incline our telescopes in like manner. By virtue ©f 
this, each star really seems to describe a small circle in 
the heavens, representing on a small scale the Earth's 
orbit ; the extent of this apparent circular motion depend- 
ing upon the relative velocity of light and of the Earth in 
its orbit, as in Fig. 87 the slope of the tube depends on 
the relative rapidity of the motion of the tube and the 
drop. From the actual dimensions of the circle, we learn 
that light travels about 10,000 times faster than the Earth 
does — that is, about 185,000 miles a second. This velocity 
has been experimentally proved by Foucault, by means of 
a turning mirror. 

411. Reflection and Refraction. — A ray of light is re- 
flected by opaque bodies which lie in its path, and is re- 
fracted, or bent out of its course, when it passes obliquely 
from a transparent medium of a certain density, such as 
air, into another of a different density, as water. 

412. Effect of Refraction. — In consequence of refrac- 
tion, the stars appear to be higher above the horizon than 
they really are. In Fig. 88, A B represents a pencil of 
light coming from a star. In its passage through our at- 

extent of the apparent .circular motion of the star depend ? From the actual 
dimensions of the circle, how fast is light found to travel ? How has Foucault 
experimentally proved this velocity ? 411. By what is a ray of light reflected ? 
Under what circumstances is it refracted ? 412. How do the stars appear, In 



REFRACTION OF LIGHT. 



229 




Fig. 88.— Illustrating Refraction. 



mosphere, since each layer 
gets denser as the surface 
of the Earth is approached, 
the ray is gradually re- 
fracted until it reaches the 
surface at G ; from which 
point the star seems to 
lie in the direction G B. 

413. The refraction of 
light can be best studied by means of a piece of glass 

with three rectan- 
gular faces, called 
a Prism. If we 
take a prism into a 
dark room, admit a 
beam of sunlight 
through a hole in 
the shutter, and let 
it fall obliquely on one of the surfaces of the prism, we 
shall see at once that the direction of the ray is changed. 
In other words, the angle at which the light falls on the 
first surface of the prism is different from the angle at 
which it leaves the second surface. 

414. Dispersion of Light. — If we receive a beam thus 
refracted by its passage through a prism, on a piece of 
smooth white paper, we shall have, instead of a spot of 
white light of the size of the hole that admitted the beam, 
a lengthened figure made up of seven different colors (as 
shown in Fig. 90), called the Spectrum. 

By passing through the prism, the beam has been 
decomposed into colored rays, occupying different places 
on account of their different degrees of refrangibility, red 




A Prism, refracting a Eat of Light. 



consequence of refraction? Illustrate this with Fig. 88. 413. How is refraction 
best studied ? How can refraction be shown with a prism ? 414. What is meant 
by the Spectrum ? Mention the colors of the spectrum in order. Why do the 
colored rays occupy different places ? Which colored ray is refracted the most? 



280 



ASTEONOMICAL INSTRUMENTS. 




Fig. 90.— The Spectkum. 

being caused to deviate the least from the course of the 
original beam, and violet the most. This separation of 
light into the different colors of the spectrum is called 
Dispersion. 

By passing the decomposed beam through a second 
prism placed in contact with the first, as in Fig. 90, the 
colored rays may be brought together again into a beam 
of white light. 

415. If we pass light through prisms of different 
materials, we shall find that, although the colors always 
maintain the same order, they will vary in length. Thus, 
if we employ a hollow prism filled with oil of cassia, we 
shall obtain a spectrum two or three times longer than if 
we use one made of common glass. This fact is expressed 
by saying that different media have different dispersive 
powers. 

Lenses. 

416. A Lens is a transparent body (commonly of glass) 
which has two polished surfaces, either both curved or one 
curved and the other plane. The general effect of lenses 
is to refract rays of light, and magnify or diminish objects 
seen through them. 

417. There are four kinds of lenses with which we have 
mainly to do ; viz., 



Which, the least? How may the colored rays he Drought together again into a 
heam of white light What is the separation of light into the colors of the 
spectrum called ? 415. If we pass light through prisms of different materials, 
what shall we find? Give an illustration. How is this fact expressed? 416. 
What is a Lens ? What is the general effect of lenses ? 417. With how many 



LENSES. 



231 



Bi-convex Lens. 
Bi-concave Lens. 
Plano-convex Lens. 
Plano-concave Lens. 




Both sides convex. 

Both sides concave. 

One side convex, the other plane. 

One side concave, the other plane. 



Fig. 91. —Different 
Kinds of Lenses. 

418. Refraction by Convex Lenses.— A prism refracts a 
ray of light as shown in Fig. 89 ; hence, two prisms ar- 
ranged as in Fig. 92 would cause two parallel beams com- 
ing from differ- 
ent points at a 
and b, to con- 
verge at one 
point c. 

We may look 
upon a bi-convex 
lens as composed 
of an infinite 
number of 
Fig. 92.— Action of two Prisms placed base to base, prisms : it will 

have a similar effect to that shown in Fig. 92. A section 





Fig. 93.— Bi-convex Lens, causing Parallel Eats to converge to a Focus. 

kinds of lenses have we mainly to do? Name and descrihe them. 418. How 
are two prisms arranged in Fig. 92 ? What is their effect on two parallel beams ? 
How may we regard a bi-convex lens ? What will be the action of such a lens on 



232 



ASTRONOMICAL INSTRUMENTS. 



of such a lens and its action on a pencil of parallel rays are 
represented in Fig. 93. All the light falling on its surface 
is refracted, and made to converge to c, which point is 
called the Focus. 

419. If we hold a common burning-glass (which is a 
bi-convex lens) up to the Sun, and let the light that passes 
through it fall on a piece of paper, the rays will be brought 
to a focus ; and if the paper is held at a certain distance 
from the lens, a hole will be burned through it. This dis- 
tance marks the Focal Distance of the lens. 

420. If we place an arrow a b in front of the bi-convex 
lens m n, we shall have an image of the arrow behind the 




Fig. 94.— Bi-convex «Lens, throwing an Inverted Image. 

lens at b a, every point of the arrow, sending a ray to every 
point in the surface of the lens. Each point of the arrow, 
in fact, is the apex of a cone of rays resting on the lens, 
and a similar cone of rays, after refraction, paints every 
point of the image. At a, for instance, in front of the lens, 
we have the apex of a cone of rays, nam / which rays, 
being refracted, form another cone of rays, n a m, behind 
the lens, painting the point a in the image. So with b, 
and so with every other point. We see that the effect of 
a bi-convex lens, like the one in the figure, is to form an 
inverted image. The line x y is called the axis of the lens. 
421. Such is the action of a bi-convex lens ; and such a 



a pencil of parallel rays? What is meant by the Focus? 419. What kind of a 
lens is a burning-glass ? What is the effect of a burning-glass ? What is meant 
by the focal distance of the lens ? 420. Explain the action of a bi-convex lens in 
forming an image. What kind of an. image does it form? 421. Explain the 



KEFKACTION BY LENSES. 



233 



lens we have in our eye. Behind it, where the image is 
cast, as in Fig. 94, we have a membrane which receives 
the image as the photographer's ground glass or prepared 
paper does ; and when the image falls on this membrane 
which is called the ret'ina, the optic nerves telegraph, as 
it were, an account of the impression to the brain, and we 
see. 

422. In order that we may see, it is essential that the 
rays should enter the eye parallel or nearly so. Hence the 
use of the common magnifying-glass. We bring the glass 
close to the eye, and place the object to be magnified in its 
focus, — that is, at c in Fig. 93 ; the rays which diverge 
from the object are rendered parallel by the lens, and we 
are enabled to see the object, which appears large because 
it is brought so close to us. 

423. Refraction by Concave Lenses. — If, instead of ar- 
ranging the prisms as shown in Fig. 92, with their bases 

together, we 
place them 
point to point, it 
is evident that 
the rays falling 
upon them will 
no longer con- 
verge ; they will 
in fact separate, 
or diverge. We 
may suppose a lens formed of an infinite number of prisms, 
joined together in this way. Such a lens is called a bi- 
concave lens. Its shape and action on parallel rays are 
shown in Fig. 95. 

424. Achromatic Lenses. — A lens being equivalent, as 
we have seen, to a combination of prisms, we would natu- 




FlG 



-Bi-concave Lens, causing Parallel Eats 
to diverge. 



principle on which we see. 422. In order that we may see, what is essential ? 
Explain the principle of the common magnifying-glass. 423. Explain the action 
of a bi-concave lens. 424. What kind of an image would we naturally expect a 



234 ASTKONOMICAL INSTRUMENTS. 

rally expect it to throw a colored image. This it does; 
and unless we could get rid of the colors, it would be im- 
possible to make a large telescope worth using. By com- 
bining, however, two lenses of different shapes, and made 
of different kinds of glass, we cause the color to disappear, 
thus forming what is called an Achromatic Lens (from the 
Greek a, without, and %pw/*a, c °Z° r ' 

425. We are able to get rid of color in the image in 
consequence of the varying dispersive powers (Art. 415) of 
different bodies. If we take two exactly similar prisms of 
the same material, and place one against the other as 
shown in Fig. 90, a beam of light passing through both 
will be unaffected ; one prism will exactly undo the work 
done by the other, and the ray will be neither refracted 
nor dispersed. But if we take away the second prism, and 
replace it with one made of a substance having a higher 
dispersive power, we shall of course be able to counteract 
the dispersive effect of the first prism with a smaller thick- 
ness of the second. 

But this smaller thickness will not counteract all the 
refractive effect of the first prism. The beam will there- 
fore leave the second prism colorless, but refracted ; and 
this is exactly what is wanted. The Chromatic Aberra- 
tion, as it is called, is corrected, but the compound prism 
can still refract. 

426. An achromatic lens is made in the same way as 
an achromatic prism. The dispersive powers of flint and 
crown glass are as .052 to .033. The front or convex lens 
is made of crown-glass. Its chromatic aberration is cor- 
rected by a bi-concave lens of flint-glass placed behind it. 
The second lens is not so concave as the first is convex ; 
hence the refractive effect of the latter is not wholly 



lens to form, and why? What would be the consequence, if we could not get 
rid of the color ? How do we get rid of it ? What is such a combination called ? 
425. How is the chromatic aberration, as it is called, corrected in the case of a 
prism ? 426. How is an achromatic lens made ? When is the spherical aberration 



THE TELESCOPE. 235 

nullified. But as the second lens equals the first in dis- 
persive power, although it cannot restore the ray to its 
original direction, it makes it colorless, or nearly so. If 
such an achromatic lens be truly made, and its curves 
properly regulated, it is said to have its spherical aberra- 
tion corrected as well as its chromatic aberration, and the 
image of a star will form a nearly colorless point at its 
focus. 

The Telescope. 

427. History.— The Telescope, to which Astronomy is 
mainly indebted for the important advances it has made 
during the last two centuries, is an instrument for viewing 
distant objects. It appears to have been invented by 
Metius, a native of Holland, in 1608. Galileo, hearing of 
the invention, constructed an instrument for himself, and 
was the first to turn the telescope to practical account. 
Since his time, many improvements have been made, 
greatly increasing the efficiency of the instrument. 

428. Construction. — The telescope is a combination of 
lenses. The principle involved in its construction is 
simply an extension of that exhibited in the structure of 
the eye. In the eye, nearly parallel rays fall on a lens, 
and this lens throws an image. In the telescope, nearly 
parallel rays fall on a lens, this lens throws an image, and 
then another lens enables the eye to form an image of the 
image by rendering the rays again parallel. These parallel 
rays enter the eye just as the rays do in ordinary vision. 

In Fig. 96, for instance, let A represent the front lens, 
called the object-glass, because it is nearest to the object 
viewed; let C represent the other, called the eye-piece, 
because it is nearest the eye; and let B represent the 
image of a distant arrow, the rays from which are seen 

said to be corrected, as well as the chromatic aberration? 427. To what is 
Astronomy mainly indebted for its recent advances ? By wbom was the tele- 
scope invented ? By wbom was it first nsed ? 428. What is the principle involved 



236 



ASTEONOM-ICAL INSTEUMENTS. 



falling on the object-glass from the 
left. These rays are refracted, and 
we get an inverted image at the focns 
of the object-glass, which is also the 
focus of the eye-piece. The rays 
leave the eye-piece adapted for vision 
as they are when they fall on the 
object-glass ; the eye can therefore 
use them as well as if no telescope 
had been there. 

429. Illuminating Power. — The 
efficiency of the telescope depends on 
two things, its Illuminating and its 
Magnifying Power. 

First, as to its Illuminating Pow- 
er. The object-glass, being larger 
than the pupil of our eye, receives 
more rays than the pupil. If its sur- 
face be a thousand times greater 
than that of the pupil, for instance, 
it receives a thousand times more 
light; and consequently the image 
of a star formed at its focus is nearly 
a thousand times brighter than that 
thrown by the lens of our eye on 
the retina. We say nearly a thou- 
sand times, because some light is lost 
by reflection from the object-glass 



and during the passage through it. F io. 96.-Constbuction 



If we have two object-glasses of the the Astronomical Tele- 
same size, one highly polished and 

the other less so, the illuminating power of the former 
will be the greater. 




in the construction of the telescope? Explain this further with Fig. 96. 429. On 
what does the efficiency of the telescope depend ? How does the telescope get 
its illuminating power? How is some of the light that falls on the object-glass 



THE TELESCOPE. 237 

430. Magnifying Power. — The Magnifying Power de- 
pends upon two things. First, it depends upon the focal 
length of the object-glass ; because, if we suppose the focus 
to lie in the circumference of a circle having its centre in 
the centre of the lens, the image will always bear the 
same proportion to the circle. Suppose it covers 1°; it is 
evident that it will be larger in a circle whose radius is 12 
feet than in one whose radius is 12 inches — that is, in the 
case of a lens whose focal length is 12 feet, than in one 
whose focal length is 12 inches. 

Next, the magnifying power of the eye-piece is to be 
taken into account. This varies according to the eye- 
piece used, the ratio of the focal length of the object-glass 
to that of the eye-piece giving its exact amount. Thus, if 
the focal length of the object-glass is 100 inches, and that of 
the eye-piece one inch, the telescope will magnify 100 times. 
But, unless the illuminating power is good and a perfect 
image is formed, a high magnifying power is useless. If 
the object-glass does not perform its part properly, the 
image will be blurred even when slightly magnified. 

431. Eye-pieces, — The eye-pieces used with the astro- 
nomical telescope vary in form. The telescope made by 
Galileo, similar in construction to the modern opera-glass, 
was furnished with a bi-concave eye-piece. This eye-piece 
is introduced between the object-glass and the focus, at a 
point where its divergent action corrects the convergent 
effect of the object-glass, and thus makes the rays parallel. 
A convex eye-piece for the same reason is placed beyond 
the focus, as shown in Fig. 96. 

Such eye-pieces, however, color the light coming from 
the image, in the same way as the object-glass would color 

lost? 430. On what two things does the magnifying power of the telescope 
depend? Show how the focal length of the object-glass has to do with the 
magnifying power. What exactly shows the amount of magnifying power? 
With any magnifying power, what is essential ? 431. Describe the eye-piece and 
its position in Galileo's telescope. What difficulty did the use of such eye-pieces 
involve ? How did Huyghens remedy this difficulty ? How are the plano-convex 



238 ASTEONOMICAL INSTRUMENTS. 

the light which forms the image, if its chromatic aberra- 
tion were not corrected. 

It was discovered by Huyghens that this defect might 
be obviated by using two plano-convex lenses, the flat 
sides toward the eye, — the larger, called the field-lens, 
nearer the image, and the smaller, called the eye-lens, 
nearer the eye. This is the construction now generally 
used except in micrometers, in which the flat sides of the 
lenses are turned away from the eye. 

432. The telescope-tube keeps the object-glass and the 
eye-piece in their proper positions; and the eye-piece is 
furnished with a draw-tube, which allows its distance from 
the object-glass to be varied. 

433. The Largest Refractor. — The largest refracting 
telescope in the world is one recently constructed in Eng- 
land, having an object-glass 25 inches in diameter. The 
pupil of the eye is ^ of an inch in diameter; this object- 
glass, therefore, will grasp over 15,000 (25 -h ^ = 125 ; 
125 2 = 15,625) times more light than the eye can. If 
used when the air is pure, it bears a power of 3,000 on the 
Moon ; in other words, the Moon seen through it appears 
as it would were it 3,000 times nearer to us, or at a dis- 
tance of 80 miles, instead of 240,000. 

434. Reflecting Telescopes.— We have thus far confined 
our attention to the principles of the ordinary astronomical 
telescope, and we have dealt with it in its simplest form. 
There are also Reflecting Telescopes, in which a speculum, 
or mirror, takes the place of the object-glass. These in- 
struments appear in several different forms. The prin- 
ciple on which Herschel's is constructed, will be under- 
stood from Fig. 97. 

The concave mirror S S is placed at the farthest ex- 
lenses turned in micrometers ? 432. What is the use of the telescope-tube ? With 
what is the eye-piece furnished ? 433. Where is the largest refracting telescope 
in the world ? What is its size ? How does the light received by the object-glass 
compare with that received by the eye ? When the air is pure, how high a power 
does it bear? 434. What other kind of telescopes is there? In reflecting tele- 




REFLECTING TELESCOPES. 239 

tremity of the 

tube, inclined so 

as to make the 

rays that fall 

upon it converge S^tp 

toward the side FlG - 97.— Principle op Herschel's Reflector. 

of the tube in which the eye-piece a b is fixed to receive 

them. The observer at E, with his back toward the 

heavenly body, looks through the eye-piece, and sees the 

reflected image. His position is such as not to prevent the 

rays from entering the open end of the tube. 

435. The Largest Reflector.— The largest reflecting 
telescope in the world is one constructed by the late Earl 
of Rosse. Its mirror is six feet in diameter, and weighs 
four tons. The tube at the bottom of which it is placed 
is fifty-two feet long and seven feet across. It is computed 
that, when this instrument is used, 250,000 times as much 
light from a heavenly body is collected as reaches the 
naked eye. 

436. Different Mountings. — An astronomer uses the 
telescope for two kinds of work : he desires to watch the 
heavenly bodies, and study their physical constitution ; he 
also wants to note their actual places and relative posi- 
tions. Accordingly, he mounts or arranges his instru- 
ment in several different ways. 

For the first kind of work the only essential is that the 
instrument should be so arranged as to command every 
portion of the sky. The best mounting for this purpose is 
shown in Fig. 98, which represents an eight-inch telescope 
equatorially mounted. With such an instrument, called an 
Equatorial, a heavenly body may be followed from its 
rising to its setting, the proper motion being communi- 

scopes, what takes the place of the object-glass? Explain the principle in 
Herschel's reflector. 435. Give an account of the largest reflector in the world. 
436. For what two kinds of work does an astronomer use the telescope ? When 
he wants to watch a heavenly body, what alone is essential ? What is the best 
mounting for this purpose ? What is an instrument so mounted called ? In 



240 



ASTEONOMICAL INSTBUMENTS. 



cated to the in- 
strument by clock- 
work. 

In this arrange- 
ment, a strong iron 
pillar supports a 
head-piece, in 
which is fixed the 
polar axis of the 
instrument paral- 
lel to the axis of 
the Earth. This 
polar axis is made 
to turn round once 
in twenty -four 
hours by the clock 
shown on the right 
of the pillar. 

It is obvious 
that a telescope 
attached to such 
an axis will always 
move in a circle 
of declination, and 
that the clock, 
turning the tele- 
scope in one direc- 

FlG. 93.-EQUATORIAL TELESCOPE. t i Qn a g f a gt a S the 

Earth is carrying it in the opposite one, will keep the in- 
strument fixed on the object. It is inconvenient to attach 
the telescope directly to the polar axis, as the range is 
then limited ; it is fixed, therefore, to a declination axis, 
placed above the polar axis and at right angles to it, as 
shown in Fig. 98. 




what will a telescope thus mounted always move ? How is the telescope kept 



MEASUEEMENT OF ANGLES. 



241 



437. For the other kinds of work, telescopes are mount- 
ed as Altazimuths, Transit-instruments, Transit-circles, and 
Zenith-sectors. 

438. Measurement of Angles. — In all these instruments, 
angles are measured by means of graduated arcs or circles 
attached to telescopes. The graduation is sometimes car- 
ried to the hundredth part of a second by Verniers, or 
small scales minutely subdivided movable by the side of 
larger fixed scales. It is of the greatest importance that 
the circle should be not only correctly graduated, but cor- 
rectly centred/ — that is, that the centre of movement should 
be also the centre of graduation. To insure greater pre- 
cision, spider-webs, or fine wires, are fixed in the focus of 
the telescope to point out the exact centre of the field of 
view. An instrument with the cross-wires perfectly ad- 
justed, is said to be correctly collimated. 

439. In addition to the fixed wires, movable ones are 
sometimes employed by which small angles may be meas- 
ured. An eye-piece so arranged is called a Micrometer. 
The movable wire is set in a frame moved by a screw, and 
the distance of this wire from the fixed central one is meas- 
ured by the number of revolutions and parts of a revolution 
of this screw, each revolution being divided into thou- 
sandths by a small circle outside the body of the microm- 
eter. 

Attached to the micrometer, or to the eye-piece which 
carries it, is also a Position-circle, divided into 360° ; by 
this the angle made by the line joining two stars, with the 
direction of movement across the field of view, is deter- 
mined. The use of the position-circle in double-star meas- 
urements is very important, and it is with its aid that their 



fixed on the same object? 437. For the other kinds of work, how are telescopes 
mounted? 438. In all these instruments, how are angles measured ? How far is 
the graduation sometimes carried, and how ? What is of the greatest impor- 
tance ? To insure greater precision, what are provided ? What is said of an 
instrument which has the cross-wires perfectly adjusted ? 439. What is meant 
by a Micrometer ? How is the movable wire fixed ? What is attached to the 



11 



242 ASTRONOMICAL INSTRUMENTS. 

orbital motion has been determined. The micrometer 
wires, or the field of view, are illuminated at night by- 
means of a small lamp outside, and a reflector inside, the 
telescope (see Fig. 99). 

440. If we want simply to measure the angular distance 
of one celestial body from another, we use a Sextant ; but, 
generally speaking, what is to be determined is not merely 
their angular distance, but their apparent position either 
on the sphere of observation or on the celestial sphere it- 
self. 

441. In the former case, — that is, when we wish to de- 
termine positions on the visible portion of the sky, — we 
use what is termed an Altitude and Azimuth Instrument, 
or briefly an Altazimuth. If we know the sidereal time, 
we can by calculation find out the right ascension and 
declination of a body whose altitude and azimuth on the 
sphere of observation we have instrumentally determined. 

442. The Altazimuth. — An altazimuth is an instrument 
with a vertical central pillar supporting a horizontal axis. 
There are two circles ; one horizontal, in which is fitted a 
smaller (ungraduated) circle with attached verniers fixed 
to the central pillar, and revolving with it ; the other, ver- 
tical, at one end of the horizontal axis, and free to move in 
all vertical planes. To this latter the telescope is fixed. 
When the telescope is directed to the south point, the 
reading of the horizontal circle is 0° ; when it is directed 
to the zenith, the reading of the vertical circle is 0°. Con- 
sequently, if we direct the telescope to any particular star, 
one circle gives the zenith distance of the star (or its alti- 
tude) ; the other gives its azimuth. 

If we fix or clamp the telescope to the vertical circle, 
we can turn the axis which carries both round, and ob- 

micrometer? What is the use of the Position-circle? How are the micrometer 
wires illuminated at night ? 440. If we want simply to measure the angular dis- 
tance of one celestial hody from another, what do we use ? Generally speaking, 
what else is to he determined ? 441. What is used when we wish to determine 
positions on the visihle portion of the sky ? 442. Descrihe the altazimuth and its 



THE ALTAZIMUTH. 



243 



serve all stars having the same altitude, and the horizontal 
circle will show their azimuths. If we clamp the axis to 
the horizontal circle, we can move the telescope so as to 




Fig. 99.— Portable Altazimuth. 

make it travel along a vertical circle, and the circle at- 
tached to the telescope will give us the zenith-distances 



244 ASTRONOMICAL INSTRUMENTS. 

of the stars (or the altitude), which, in this case, will lie in 
two azimuths 180° apart. 

Fig. 99 represents a portable altazimuth, the various 
parts of which will be recognized from the foregoing de- 
scription. 

443. The Transit-circle. — When we wish to determine 
directly the position of a heavenly body on the celestial 
sphere, a Transit-circle is used. This instrument consists 
of a telescope movable in the plane of the meridian, being 
supported on two pillars, east and west, by means of a 
horizontal axis. The ends of the axis are of exactly equal 
size, and move in pieces, which, from their shape, are 
called Y's. When the instrument is in perfect adjustment, 
the line of collimation of the telescope is at right angles to 
the axis, the axis is exactly horizontal, and its ends are 
due east and west. Under these conditions, the telescope 
describes a great circle of the heavens, passing through 
the north and south points and the celestial pole ; that is, 
in all positions it points to some part of the meridian of 
the place. 

On one side of the telescope is fixed a circle, which is 
read by microscopes attached to one of the supporting 
pillars. The cross-wires in the eye-piece of the telescope 
enable us to determine the exact moment of sidereal time 
at which the meridian is crossed ; this time is the right 
ascension of the object. The circle attached shows us its 
distance from the celestial equator ; this is its declination. 
So by one observation, if the clock is right, the instrument 
perfectly adjusted, and the circle correctly divided, we 
get both coordinates. 

444. Determination of Positions with the Transit-circle. 
—As we have already seen, a celestial meridian is nothing 



mode of operation. 443. When we wish to determine directly the position of a 
heavenly hody on the celestial sphere, what is used? Of what does the transit- 
circle consist ? When the instrument is peiiectly adjusted, what does the tele- 
scope describe ? How are right ascension and declination found with the transit- 



THE TEANSIT-CIKCLE. 245 

but the extension of a terrestrial one; and as the latter 
passes through the poles of the Earth, the former will pass 
through the poles of the celestial sphere: consequently, 
wherever we may be, the northern celestial pole will lie 
somewhere in the plane of our meridian. If the position 
of the pole were exactly marked by the pole-star, this star 
would remain immovable in the meridian; and when a 
celestial body was also in the meridian, if we adjusted the 
circle so as to read 0° when the telescope pointed to the 
pole, we could determine the north-polar distance of the 
body by simply pointing the telescope to it, and noting 
the angular distance shown by the circle. 

But, as the pole-star does not lie exactly at the pole, 
we have to adopt some other method. We observe the 
zenith-distance of a circumpolar star when it passes the 
meridian above the pole, and also when it passes below 
it, and taking half the sum of these zenith-distances, we 
find the zenith-distance of the celestial pole. The celestial 
equator, which is 90° from the celestial pole, can then be 
readily determined ; its zenith-distance will be the dif- 
ference between the zenith-distance of the celestial pole, 
already known, and 90°. The horizon, which is 90° from 
the zenith, can also be determined. We can, therefore, 
measure angular distances with our transit-circle, 
I. From the zenith. 
II. From the celestial pole. 

III. From the celestial equator. 

IV. From the horizon. 

Any of these distances can easily be turned into any 
other. 

445. When we have obtained the distance from the 
celestial equator, we get in the heavens the equivalent of 

circle? 444. If the celestial pole exactly corresponded with the polar star, how 
could we determine the north-polar distance of a body ? As it is, how do we find 
the north celestial pole ? What will the zenith-distance of the celestial equator 
he equal to ? How can the horizon he determined ? From what, therefore, may 
we measure angular distances with the transit-circle? 445. What is distance 



246 ASTRONOMICAL INSTRUMENTS. 

terrestrial latitude. But this is not enough; a hundred 
places may have the same latitude, a hundred stars may- 
have the same declination ; we need what is called another 
coordinate, to fix their position. On the Earth we get 
this other coordinate by reckoning from the meridian 
which passes through the centre of the transit-circle at 
Greenwich. So in the heavens we reckon from the posi- 
tion occupied by the Sun at the vernal equinox. 

The astronomer has, not only a telescope and circle, 
but also a sidereal clock, adjusted (as already stated) to 
the apparent movement of the stars, or the actual rota- 
tion of the Earth. Sidereal time, like right ascension, is 
reckoned from the first point of Aries. Hence, a sidereal 
clock at any place will denote the right ascension of the 
celestial meridian visible in the transit-circle at that 
moment ; and if we at the same moment, by means of the 
circle, note how far a heavenly body is from the celestial 
equator, we shall know both its right ascension and declina- 
tion, and its place in the heavens will be determined. The 
Earth itself, by its rotation, brings every star in turn to 
the meridian of our place of observation, and thus per- 
forms the most difficult part of the work for us. 

446. In order that the angular distance from the 
zenith, and the time of meridian passage, may be correctly 
determined, observations of the utmost delicacy are re- 
quired. 

The circle of the transit used at Greenwich is read by 
the microscopes in six different parts of the limb at each 
observation, and the recorded zenith-distance is the mean 
of these readings. The right ascension is obtained with 
equal care. The transit of the star is watched over nine 
equidistant wires, in the micrometer eye-piece (called in 



from the celestial equator called? What besides declination is needed, to deter- 
mine a heavenly body's position ? How is right ascension obtained ? How does 
the Earth itself assist us in finding it ? 446. What facts are mentioned, to show 
the care with which observations are made ? 447. How many methods are there 



*^ 



THE TEANSIT-CIECLE. 247 

this case a transit eye-piece) , the middle one being exactly 
in the axis of the telescope. 

447. Methods of determining the Time of Transit over 
a Wire. — There are two methods of observing the time of 
transit over a wire, one called the eye and ear method, the 
other the galvanic or chrono graphic method. In the 
former, the observer, taking his time from the sidereal 
clock, which is always close to the transit-circle, listens to 
the beats, and estimates at what interval between two 
beats the star passes behind each wire. An experienced 
observer mentally divides a second of time into ten equal 
parts with no great effort. 

In the second method, an apparatus called a Chrono- 
graph is used. A barrel covered with paper is made to 
revolve at a uniform rate of speed. By means of a gal- 
vanic current, a pricker attached to the keeper of an 
electro-magnet is made at each beat of the sidereal clock 
to puncture the revolving barrel. The pricker is carried 
along the barrel, so that the punctures, about half an inch 
apart, form a spiral, Here, then, we have the flow of time 
fairly recorded on the barrel. At the beginning of each 
minute the clock fails to send the current, so that there is 
no confusion. What the clock does regularly at each beat, 
the observer does when a star crosses the wires of his 
transit eye-piece. He presses a spring, and an additional 
current at once makes a puncture on the barrel. The time 
at which the transit of each wire has been effected, is esti- 
mated from the position the additional puncture occupies 
between the punctures made by the clock at intervals of a 
second. 

The observer is thus enabled to confine his attention to 
the star. After completing his observation, he can at 
leisure make the necessary notes on the punctured paper 

of determining the time of transit over a wire ? What are they called ? Describe 
"the eye and ear method.' 1 What is used in the second method? Give an 
account of the mode of using the chronograph. What advantage has the observer 



248 ASTRONOMICAL INSTRUMENTS. 

which is taken off the barrel when filled, and bound up as 
a permanent record. 

448. Determination of Positions with the Equatorial. — 
With the transit-circle, the position of a body in the celes- 
tial sphere can be determined only when it is on the merid- 
ian. The equatorial enables this to be done, on the other 
hand, in every part of the sky, though not with such ex- 
treme precision. The object is brought to the cross-wires 
of the micrometer eye-piece, and the declination-circle at 
once shows its declination. The right ascension is deter- 
mined as follows : — At the lower end of the polar axis is a 
movable circle divided into the 24 hours. Flush with the 
graduation are two verniers ; the upper one fixed to the 
stand, the lower one movable with the telescope. The 
fixed vernier shows the position occupied by the telescope, 
and therefore by the movable vernier, when the telescope 
is exactly in the meridian. Prior to the observation, the 
circle is adjusted so that the local sidereal time — or the 
right ascension of the part of the celestial sphere in the 
meridian — is brought to the fixed vernier. The circle is 
then moved by the clockwork of the instrument ; and when 
the cross-wires of the telescope are adjusted on the object, 
the movable vernier shows its right ascension on the same 
circle. 

449. Star-catalogues. — The method which is good for 
determining the exact place of a single heavenly body is 
good for mapping the entire heavens; accordingly, the 
whole celestial sphere has been mapped out, the right as- 
cension and declination of every object having been deter- 
mined. 

The most important of the catalogues in which these 
positions are contained, is due to the German astronomer 

in this method ? 448. As regards the determination of positions, how does the 
equatorial differ from the transit-circle? How is declination obtained with the 
equatorial ? How is right ascension determined ? 449. What has been accom- 
plished through these methods of finding the declination and right ascension ? 



COEEECTION OF OBSERVATIONS. 249 

Argelander. This catalogue contains the positions of up- 
ward of 324,000 stars, from 1ST. Decl. 90° to S. Decl. 2°. 
Bessel also has put forth a catalogue of more than 32,000 
stars. Airy and the British Association have published 
similar lists. There are also catalogues dealing with 
double and variable stars exclusively. 

450. Corrections to be applied. — After the astronomer 
has made his observations of a heavenly body, and has 
freed them from instrumental and clock errors, he has ob- - 
tained what is termed the observed or apparent place. 
This, however, is worth very little ; he must, in order to 
obtain its true place, apply other corrections. 

451. Correction for Refraction. — The first correction is 
needed, to nullify the effect of refraction already explained. 
Refraction causes a heavenly body to appear higher the 
nearer it is to the horizon. On an object in the zenifh it 
has no effect : on one near the horizon, its effect is very 
decided ; at sunset, for instance, in consequence of refrac- 
tion, the Sun appears above the horizon after it has actually 
sunk below it. 

The correction, therefore, to be made for refraction, 
depends entirely on the altitude of the body on the sphere 
of observation. Table VII. in the Appendix shows the 
amount of correction for different altitudes. In practice, 
the corrections are themselves corrected according to the 
density of the air at the time of observation. 

452. Correction for Aberration. — We have already al- 
luded to the aberration of light (Art. 410). It results from 
the fact that the observer's telescope, carried round by the 
Earth's annual motion round the Sun, must always be 
pointed a little in advance of the star, in order, as it were, 

By whom have star-catalogues been published ? 450. After the astronomer has 
found the apparent place of a heavenly body, what has he yet to do ? 451. For 
what is the first correction needed? What is the effect of refraction on a 
heavenly body in different positions ? On what, therefore, does the correction 
to be made for refraction entirely depend ? In practice, according to what are 
the corrections themselves corrected? 452. For what is the next correction to 




EARTH'S WAY 



250 ASTKONOMICAL INSTRUMENTS. 

to catch the light from it. Hence the star's aberration- 
place will be different from its real place y and, as the 
Earth travels round the Sun, and the telescope is carried 
round with it always pointed ahead of the star's place, the 

aberration-p lace r e - 

C ' f volves round the real 

<35 5 place exactly as the 

Earth (if its orbit be 

_, inn _ ' """. "" . 7 regarded as circular) 

Fig. 100.— Effect of Aberration : a, b, c, d, & ' 

the Earth in different parts of its orbit ; a', would be Seen from the 
V c', d', the corresponding aherration-places , . rpvo ] vp rmiTlf I 

of the star, varying from the true place in the bldjL lu ievuivy iouuu 
direction of the Earth's motion at the time. the Sun. The aberra- 

tion-places of all stars, in fact, describe circles parallel to 
the plane of the Earth's orbit. If the star lie at the pole 
of the ecliptic, the path of its aberration-place will appear 
as a circle, the centre of which will be at its true place. 
The aberration-place of a star in the ecliptic will oscillate 
backward and forward, as we are in the plane of the circle ; 
that of one in a middle celestial latitude will appear to 
describe an ellipse. The diameter of the circle, the major 
axis of the ellipse, and the amount of oscillation, will all 
be equal — about 40.5" ; but the minor axis of the ellipses 
described by the stars in middle latitudes will increase from 
the equator to the pole. The correction to be made is half 
of the above-mentioned invariable quantity, or 20.25", 
which is called the constant of aberration. It is deter- 
mined by the following proportion, bearing in mind that 
the 360° of the Earth's orbit are passed over in 365^ days, 
and that light takes about 8 minutes 13 seconds to come 
from the Sun : — 

Days. m. s. ° " 

365^ : 8 13 :: 360 : 20.25 
453. The direction of the Earth's motion in its orbit, 

be made? Prom what does aberration result? How docs the aberration-place 
move, in the case of stars in different positions ? What is the allowance to be 
made for aberration? What is it called? How is the constant of aberration 
determined ? 453. What is meant by the Earth's way ? How far is it from the 



PAEALLAX. 251 

called the Earth's way, referred to the ecliptic, is always 
90° behind the Sun's position in the ecliptic at the time*; 
therefore the aberration-place of a star will lie on' the great 
circle passing through the star and the point in the ecliptic 
90° behind the Sun. 

454. Correction for Parallax. — Observations of the ce- 
lestial bodies comparatively near the Earth, such' as the 
Moon and some of the planets, when made at different 
places on the Earth's surface, though corrected as we have 
indicated, do not give the same result, as their positions on 
the celestial sphere appear different to observers at differ- 
ent points of the Earth's surface. This effect will be readily 
understood by changing our position with regard to any 
near object, and observing it as projected on different 
backgrounds in the landscape. The nearer we are to the 
object, the more will its position appear to change. To 
get rid of these discrepancies, the observed positions are 
further corrected to what they would be were the observa- 
tions made at the centre of the Earth. This is called ap- 
plying the correction for parallax. 

455. Parallax is the angle under which a line drawn 
from the observer to the centre of the Earth would appear 
at the body observed; in other words, it is the angle 
formed at the body in question by two lines drawn one to 
the observer's eye and the other to the Earth's centre. 
When a body is at the zenith, it has no parallax. When 
it is on the horizon, its parallax, which is then termed its 
Horizontal Parallax, is greatest. 

This is obvious from Fig. 101. C being the Earth's 
centre, and an observer, a body at Z (the zenith) is seen 
in exactly the same direction from both points, and has 
no parallax. At S its parallax is 08 0, and at H it is 

Sun's position in the ecliptic ? In what, therefore, will the aherration-place of a 
star lie ? 454. Why do observations made at different parts of the Earth's surface 
have to be corrected to what they would be if made from the Earth's centre ? 
What is this correction called? 455. What is Parallax? What is Horizontal 
Parallax ? Where is parallax least, and where greatest ? Show this with Pig. 



252 



ASTKONOMICAL INSTKUMENTS. 




Fig. 101.— Parallax. 



OHO, which is greater 
than OS C or the angle 
that would be formed 
at any point between S 
and IT. 

As shown by the 
9 dotted prolongations of 
the lines OS, C S, a 
body seen from would 
appear farther from Z 
than if seen from C; 
hence, to obtain the true 
zenith-distance, we must 
subtract the correction 
for parallax from the ap- 
parent zenith-distance. 

456. Changes in Positions already determined. — We 
have seen that the positions of the heavenly bodies are 
determined with reference to either the plane of the 
ecliptic or the plane of the equator; and that from one 
of the points of intersection of these two planes — that, 
namely, occupied by the Sun at the vernal equinox, called 
the first point of Aries and written T — right ascension 
and celestial longitude are both reckoned. If these planes, 
then, are changeless, a position once determined will be 
determined forever; but if either plane varies, then the 
point of intersection will of course vary, and corrections in 
the positions of the stars as once determined will be neces- 
sary from time to time. Now, it is found that changes 
occur in both planes. 

457. It has been stated that the Earth's axis always 
points in the same direction. Strictly speaking, this is 



101. How must the correction for parallax be used, to obtain the true zenith- 
distance ? 456. What renders corrections in the positions of the stars, as once 
determined, necessary from time to time? 457. What change takes place in the 
position of the Earth's pole ? From this what important fact follows ? How does 



PEECESSION OF THE EQUINOXES. . 253 

not the case. The pole of the Earth is constantly changing 
its position, and revolves round the pole of the ecliptic in 
25,868 years, so that the pole-star of to-day will not be the 
pole-star 3,000 years hence. 

From this a very important fact follows. As the Earth's 
axis changes, the plane of the equator changes with it, and 
so that each succeeding vernal equinox happens a little 
earlier than it would otherwise do. This is called the 
Precession of the Equinoxes, because the equinox seems to 
move backward, or from left to right, so as to meet the 
Sun earlier. In the time of Hipparchus — 2,000 years ago 
— the Sun at the vernal equinox was in the constellation 
Aries ; it is now in the constellation Pisces. 

458. The plane of the ecliptic is also subject to varia- 
tion. This is termed the Secular Variation of the Obliquity 
of the Ecliptic. 

459. Of these changes, the precession of the equinoxes 
is the more important. It causes the point of intersection 
of the two fundamental planes to recede 50.37572" annually. 
To this is due the difference in length between the sidereal 
and the tropical year. 

460. Cause and Effect of these Changes. — The cause of 
these changes is the attraction exercised by the Sun, 
Moon, and planets, upon the protuberant equatorial por- 
tions of our Earth. The effect is to render both latitudes 
and longitudes, right ascensions and declinations, variable. 
Hence the observed position of a heavenly body to-day 
will not be the position occupied last year, or to be 
occupied next year. Apparent positions have to be cor- 
rected, to bring them to some common epoch, such as 
1850, 1880, etc., so that they may be strictly comparable. 

the equinox seem to move? What is this motion called? Since the time of 
Hipparchus, what change has taken place in the position of the Sun at the vernal 
equinox ? 458. What is the variation in the plane of the ecliptic called ? 459. Of 
these changes, which is the more important ? What is the amount of recession 
annually? What does this recession cause? 460. What is the cause of these 
changes in the plane of the ecliptic and the plane of the equator ? What is their 



254 ASTKONOMICAL INSTRUMENTS. 

461. Celestial Latitude and Longitude, how determined. 

— Celestial latitude and longitude, which are used to 
determine the position of heavenly bodies with reference 
to the ecliptic, are not obtained by observation, but are 
calculated from the true right ascension and declination 
by means of spherical trigonometry. 

462. Recapitulation. — Let us recapitulate what has 
been said as to the methods by which the true positions 
of the heavenly bodies are obtained : — 

1. The astronomer, to make observations on that part 
of the celestial sphere which is visible to him, makes use 
principally of a sextant or an altazimuth. The positions 
of a body thus determined may by calculation be referred 
to- the celestial sphere itself, and its right ascension and 
declination determined. 

2. Observations of a body with regard to the celestial 
sphere itself are made principally by means of a transit- 
circle or an equatorial, by which both apparent right 
ascension and declination may be directly determined. 

3. In all observations, the instrumental and clock errors 
are carefully corrected. 

4. Besides the instrumental and clock errors, there are 
others — caused by the refraction and aberration of light, 
—which must also be corrected. 

5. Besides these, another error, parallax, results from 
the observer's position on the Earth's surface. This is 
corrected by reducing all observations to the Earth's 
centre. 

6. There are still other errors depending upon the 
change of the intersection of the two planes to which all 
measurements are referred. These are got rid of by re- 
ducing all observations to a point of time (as parallax was 
got rid of by reducing them to a point of space — the centre 

effect? To what do apparent positions have to be corrected? 461. How are 
celestial latitude and longitude obtained? 462. Eecapitulate what has been said 
as to the methods by which the true positions of the heavenly bodies are 



NAUTICAL ALMANACS. 255 

of the Earth). Some year is fixed upon, and the observa- 
tions reduced to what they would have been at this time 
if the year is past, or what they will be when made at this 
time if the year is to come. 

1. The right ascension and declination are easily con- 
verted by calculation into celestial longitude and latitude, 
if required. 

463. By means of observations freed from these errors, 
and extending over centuries, astronomers have been able 
to determine the positions of all the stars with the greatest 
accuracy, and to discover the proper motions of some of 
their number. They have also investigated the motions 
of the bodies of our system so thoroughly as to ascertain 
the laws by which they are regulated, and to be able to 
predict their exact positions for years to come. 

This information is embodied in an Almanac or Ephe- 
meris, published in advance by each of the principal Gov- 
ernments, for the use of travellers and navigators. The 
United States and the English Nautical Almanac are pub- 
lications of this character, in which are given, with most 
minute accuracy, the positions of the principal stars, the 
planets, and the Sun, from day to day, and the positions 
of the Moon from hour to hour. These positions enable us 
to determine — I. Time. II. Latitude. III. Longitude. 

Determination of Time, Latitude, and Longitude. 

464. Determination of Time. — When time only is re- 
quired, a transit-instrument is used ; that is, a simple tel- 
escope mounted like the transit-circle, but without the circle, 
or with only a small one — the transits of stars, the right 
ascension of which has been already determined with great 
accuracy by transit-circles, in fixed observatories, being 

obtained. 483. By means of observations thus freed from errors and extending over 
centuries, what have astronomers been able to do? In what is this information 
embodied ? What are given in the Nautical Almanac ? What do these positions 
enable us to determine ? 464. When time only is required, what is used? How 



256 ASTRONOMICAL INSTRUMENTS. 

observed. This gives us the local sidereal time, which 
may, if necessary, be converted into mean solar time. 

465. Determination of Latitude. — To determine our 
position on the Earth's surface, all we need is our latitude 
and longitude. The determination of the former in a fixed 
observatory is an easy matter, if proper instruments be at 
hand. For instance : half the sum of the altitudes (cor- 
rected for refraction) of a circumpolar star, at upper and 
lower culmination, even if its position is unknown, will 
give us the elevation of the pole, and therefore the latitude 
of the place. 

Again, if we find the zenith-distance of a star, the dec- 
lination of which has been accurately determined, we can 
readily obtain the latitude. For, as declination is referred 
to the plane of the terrestrial equator prolonged to the 
stars, it is the exact equivalent of terrestrial latitude. If 
a star of 0° declination is observed exactly in the zenith, 
the observer must be on the equator ; if the declination of 
a star in the zenith is 45°, then our latitude is 45° ; if a 
star of declination 39° N". passes 10" to the north of our 
zenith, then our latitude is 38° 59' 50", and so on. 

466. On board ship, and in the case of explorers, the 
problem is for the most part limited to determining the 
meridian altitude of the Sun or Moon, as the sextant only 
can be employed. Suppose such an observation to give the 
altitude as 29° from the south point of the horizon — equiv- 
alent to 61° zenith-distance — and that the Nautical Al- 
manac gives its declination on that day as 1 2° south ; if 
we were in lat. 12° S. the Sun would be overhead, and its 
zenith-distance would be 0° ; as it is 61° to the south, we 
are 61° to the north of 12° S., or in N. lat. 49°. So, if we. 
find the meridian altitude to be 10° from the north point 
of the horizon (or the zenith-distance to be 80°), and the 

is the local sidereal time obtained? 465. What do we need, to. determine our 
position on the Earth's surface ? In what two ways can we find the latitude in a 
fixed observatory ? Illustrate the latter method. 466. How can latitude be deter- 



DETEEMINAT10N OF LONGITUDE. 2 57 

Nautical Almanac gives the declination at the time as 20° 
N., our position will be in 60° S. lat. 

467. Determination of Longitude.— Longitude is in fact 
time, and difference of longitude is the difference of the 
times at which the Sun crosses any two meridians, the 
twenty-four hours solar mean time being distributed among 
the 360° of longitude, so that 1 hour = 15 , and so on. 

Several ways of determining longitude are employed 
in fixed observatories. The most convenient one consists 
in electrically connecting the two stations whose difference 
of longitude is sought, and observing the transit of the 
same stars at each. Thus the transits at station A are re- 
corded on the chronograph at stations A and B, and the 
transits at station B are similarly recorded at B and A ; 
from both chronographs the interval between the times of 
transit is accurately recorded in sidereal time, and the 
mean of all the differences converted into mean solar time 
gives the difference of longitude. 

468. One mode of determining longitude at sea, which 
consists in finding the difference between local time, and 
Greenwich time as indicated by an accurate chronometer, 
and converting this difference into difference of longitude, 
has been explained in Art. 191. 

A second method consists in making use of the heavens 
as a dial-plate, and of the Moon as the hand. In the 
Nautical Almanac, the distances of the Moon from the stars 
in her course are given, as they would appear if observed 
from the Earth's centre, for every third hour in Green- 
wich time. The sailor, therefore, observes the Moon's 
distance from the stars in question, and corrects his obser- 
vation for refraction and parallax. Referring to the 
Nautical Almanac, he sees the time at Greenwich at which 



mined at sea ? Give examples. 467. To what is longitude, and to what is dif- 
ference of longitude, really equivalent? What is the most convenient mode of 
determining longitude in fixed observatories ? 468. What method of determining 
longitude at sea has already been explained? What other method is there? 



258 ASTRONOMICAL INSTRUMENTS. 

the distance is the same as that which he has obtained ; 
and knowing the local time (from day observations) at the 
instant at which his observation was made, he readily finds 
the difference of time, and thence the difference of longi- 
tude. 

Determination of Distances. 

469. To determine the distances of the heavenly bodies, 
astronomers have recourse to methods similar to those used 
by surveyors in measuring distances on the Earth. When 
two angles of a triangle and the length of the included side 
are known, the remaining angle and sides can with the aid 
of trigonometry be determined. Accordingly, a base-line 
which subtends an appreciable angle at the body in ques- 
tion, and whose length is accurately found, being taken, a 
triangle is formed by drawing lines from the ends of the 
base to the object. The angles at the extremities of the 
base-line being then determined by observation, the par- 
allax of the body and its distance from the Earth can be 
found. 

470. Parallax of the Moon. — In the case of the Moon, 
the base-line taken is the distance between two places on 
the Earth's surface remote from each other, which distance 
can be determined from their positions on the globe when 
the size of the Earth is known. The mean equatorial hori- 
zontal parallax of the Moon has thus been found to be 
nearly 51' 3". 

471. Determination of the Distance of Mars. — In the 
case of Mars, the Earth's diameter is made the base-line, 
observations being taken at the same place at an interval 
of 12 hours, which owing to the Earth's rotation separates 
the points of observation by the length of the Earth's 



469. To determine the distances of the heavenly bodies, to what do astronomers 
have recourse? Explain the mode of proceeding. 470. What is taken for a base- 
line in the case of the Moon ? What is the mean equatorial horizontal parallax 
of the Moon found to be ? 471. What is made the base-line in the case of Mars ? 



THE SUN'S PAKALLAX. 259 

diameter — allowance being made for the motion of both 
planets in the interval between the observations. 

472. The Sun's Parallax. — As seen from the Sun, the 
Earth's diameter is so small that it is useless as a base-line 
in determining the Sun's distance. This can, however, be 
obtained directly by a method pointed out by Halley in 
1716, based on observations of the transit of Venus. Un- 
fortunately, these transits happen but rarely ; the last took 
place in 1874, the next available one will be in 1882. On 
the other hand, when they do occur, as the planet is pro- 
jected on the Sun, the Sun serves the purpose of a microm- 
eter, and observations may be made with the most rigor- 
ous exactness. 



The old value of the Sun's parallax, obtained by 

Bessel from the transit of Venus, was . 8.578" 

New value obtained by Hansen from the Moon's 

parallactic equation, 8.916" 
" " Winnecke from observa- 

tions of Mars, . . . 8.964" 

" " Stone, 8.930" 

" " Foucault, from the veloci- 

ty of light, . . . 8.960" 
" " Le Verrier, from the mo- 

tions of Mars, Venus, 
and the Moon, . . . 8.950" 

The difference between the old and the new value now 
generally accepted, which equals about two-fifths of a sec- 
ond of arc, amounts to no more than the apparent breadth 
of a human hair viewed at the distance of about 125 feet. 
Yet it requires us to alter the distance and diameter of 
nearly every body in the solar system, and makes a differ- 

How is this done ? 472. On what is the method Of finding the Sun's parallax 
based? Why is not the Earth's diameter used as a base-line? When will the 
next transit of Venus occur? What was the old value of the Sun's parallax, 
obtained from the transit of Venus? What later values have been obtained? 
What is the difference between the old and the new value now generally accepted ? 



260 ASTRONOMICAL INSTRUMENTS. 

ence of about 3J- millions of miles in the distance of the 
Sun itself. According to the old value of the parallax, the 
Sun's distance was about 95,000,000 miles ; according to 
the new, it is but 91,430,000. The transit of 1874 was 
observed with the greatest care, to see whether this new 
value of the solar parallax would be confirmed. 

473. Parallax of the Stars.— Having thus obtained the 
distance of the Sun, we have a base-line of enormous di- 
mensions ; for the positions successively occupied by our 
Earth in two opposite points of its orbit will be 183,000,000 
miles apart, and we can make this our base-line by taking 
observations at the same place at an interval of six months. 
But we find that even this great line is insufficient to meas- 
ure the distances of the stars. In almost every case, there 
is no apparent difference in the position as observed in 
January and July, February and August, etc. As seen 
from the fixed stars, the Earth's orbit is but a point ! 

Now, an instrument such as is ordinarily used should 
show us a parallax of one second — that is, an angle of \" 
subtended at the star by half the base-line we are using ; 
and a parallax of 1" means that the object is 206,265 times 
farther away than we are from the Sun, as the Sun's dis- 
tance is the half of our base-line. If, then, a star's parallax 
be less than 1", the star must be farther away than 206,265 
times 91,430,000 miles ! — and this we find to be the case 
with every star in the heavens. 

474. In the great majority of cases, the true zenith- 
distance of a star is the same all the year round. As this 
true place results from the several corrections referred to 
in Art. 462, even when there is a slight variation, it may 
be wrong to ascribe it to parallax. A slight error in the 

What difference in the Sun's distance does this small difference of parallax make ? 
473. What may now be taken as a base-line, to find the parallax of the stars ? 
How may it he made available ? Is it found sufficient for the purpose ? Why 
not? How great a parallax should an ordinary telescope show us? If a star's 
parallax be less than one second, how far must the star be away? What follows 
with respect to the distance of every star in the heavens ? 474. In the case of 
what star alone was the parallax found by this method? What method was 



PAEALLAXES OF THE STAES. 



261 



refraction, or the presence of proper motion in the star, 
would give rise to a greater difference of position than the 
one due to parallax, as in no case does the latter exceed 1". 
Hence, as long as the problem was approached in this 
manner, very little progress was made, the parallax of a 
Centaur i (0.9187") alone being obtained. 

Bessel, however, employed a method by which the 
various corrections were done away with, or nearly so. 
He chose a star having a decided proper motion, and com- 
pared its position, night after night, by means of the 
micrometer only, with other small stars lying near it which 
had no proper motion, and which therefore he assumed to 
be very much farther away. He found that the star "with 
the proper motion did really change its position with re- 
gard to the more remote ones, as it was observed from dif- 
ferent parts of the Earth's orbit. This method has since 
been pursued with great success. Here is a table showing 
the parallax and distance of some of the nearer stars, as 
obtained by this method. 



Star. 



a Centauri . . 

61 Cygni . . 
1830 Groombridge 

70 Ophiuchi . 

Vega . . . 

Sirius . . . 

Arcturus . . 

Polaris . . , 

Capella . . . 



Parallax. 



0.9187 

0.5638 

0.226 

0.16 

0.155 

0.15 

0.127 

0.067 

0.046 



Distance. 

Sun's distance 

= 1. 



224,000 
366,000 
912,000 
1,286,000 
1,337,000 
1,375,000 
1,624,000 
3,078,000 
4,484,000 



employed by Bessel ? How did it succeed ? What star is nearest lo the Earth ? 
What is its parallax, and what its distance in terms of the Sun's distance ? 
What is the next nearest star? Give the parallax and distance of 61 Cygni. Of 



262 ASTEONOMICAL INSTRUMENTS. 

Thus a Centaury which is the nearest star, is found to 
be 224,000 times as far off as the Sun, or more than 
20,000,000,000,000 miles. 

Determination of the Size of the Heavenly Bodies. 

475. When the distance of a body is known, and also 
its angular measurement, its size is determined by a simple 
proportion ; for the distance is, in fact, the radius of the 
circle on which the angle is measured. 

There are 1,296,000 (360X60X60) seconds in an entire 
circumference. Hence, as the circumference is 3.1416 
times the diameter, and the diameter is twice the radius, 
there are as many seconds in that part of the circumference 
which equals the radius as twice 3.1416 is contained times 
in 1,296,000 — or 206,265". Hence the following propor- 
tion : — 



The diameter ) ( ,, flistaT1PP ) ( the angular ) ( 

of the bocly \ : ] [ Smiles f :: 1 diameter [ : < 

in miles ) ( m miles j ( in seconds, j j 



20026!; 



Calling the diameter in miles d, by multiplying the means 
together and dividing their product by the given extreme, 
we get the following formula : — 

r. _ distance x angular diameter ,-.-. 

! 206265 ' • ' ■ " * ' . } 

The mean angular diameter of the Moon is 31' 8.8", or 18C8.8" ; its 
distance is 23*7,640 miles ; what is its diameter in miles ? 
Applying Formula 1, we have 

d = 237640 x 18G8.8 = ^ ^ 
206265 
In Table II. of the Appendix are given the greatest and least apparent 
angular diameters of the planets, as seen from the Earth. Hence 
the mean angular diameters can be found, and with these and the dis- 
tances given in Art. 367, the student can calculate the real diameters for 
himself. 



Vega. OfSirius. Of Arctarus. 475. From what can the size of a heavenly body 
be determined, and how ? Give the process by which the formula for finding the 
diameter in miles can be obtained. Give the formula. Apply this formula, to 



THE SPECTRUM. 263 

476. From Formula 1 we derive Formula 2 given 
below, which is to be used when the diameter in miles and 
the angular diameter are known, and the distance is re- 
quired : — 

Distance = 20626S * * . . (2 ) 

angular diameter in seconds 

The diameter of the Sun being 852,584 miles, and its mean angular 
diameter 32' 4.205", what is its distance? 



CHAPTER XV. 
THE SPECTRUM. 

477. A caeeful examination of the solar spectrum has 
revealed to us the importance of solar radiation (Art. 127). 
Not only may we liken the gloriously-colored bands which 
we call the spectrum to the key-board of an organ — each 
ray a note, each variation in color a variation in pitch — 
but as there are sounds in nature which we cannot hear, 
so there are rays in the sunbeam which we cannot see. 

What we do see is a band of color extending from 
red, through orange, yellow, green, blue, and indigo, to 
violet ; but at either end the spectrum is continued. There 
are dark rays before we get to the red, and other dark 
rays after we leave the violet — the former heat rays, the 
latter chemical rays. .This accounts for the threefold 
action of the sunbeam; heating power, lighting power, 
and chemical power. 

478. Gradual Formation of a Spectrum. — When a cool 
body, such as a poker, is heated in the fire, the rays it 

find the diameter of the Moon. 476. Give the formula for finding the distance, 
when the diameter in miles and the angular diameter are known. Apply this 
formula, to find the distance of the Sun. 

477. What has been revealed to us by an examination of the solar spectrum ? 
What do we see in the spectrum ? What are there that we do not see ? What 
three kinds of rays are combined in the sunbeam ? 478. Give an account of the. 



264 " THE SPECTEUM. 

first emits are invisible ; if we look at it through a prism, 
we see nothing, though we easily perceive by the hand 
that it is radiating heat. As it is more highly heated, the 
radiation gradually increases, until the poker becomes of a 
dull-red color, the first sign of incandescence ; in addition 
to the dark rays it previously emitted, it now sends forth 
waves of red light, which a prism will show at the red end 
of the spectrum. If we still increase the heat and con- 
tinue to look through the prism, we find, added to the 
red, orange, then yellow, then green, then blue, indigo, 
and violet ; when the poker is white-hot, all the colors of 
the spectrum are present. If, after this point has been 
reached, the substance allows of still greater heating, it 
will give out with increasing intensity the rays beyond 
the violet, until the glowing body can rapidly act in form- 
ing chemical combinations, a process which requires rays 
of the highest refrangibility — the so-called chemical, ac- 
tinic, or ultra-violet rays. 

479. Fraunhofer's Lines. — We owe the discovery of 
the prismatic spectrum to Sir Isaac Newton, but the 
beautiful coloring is but one part of it. Dr. Wollaston, in 
1802, discovered that there were dark lines crossing the 
spectrum in different places. These have been called 
Fraunhofer's Lines, as an eminent German optician of that 
name afterward mapped the plainest of them with great 
care ; he also discovered that there were similar lines in 
the spectra of the stars. The explanation of these dark 
lines we owe mainly to Kirchhoff. The law which ex- 
plains them was, however, first proved by Balfour Stewart. 

480. Experiments with the Spectroscope. — We shall 
observe the lines best if we make our sunbeam pass 
through an instrument called a Spectroscope, in which 



puccessive steps in the formation of a spectrum by a body subjected to heat. 479. 
By whom was the prismatic spectrum discovered? What discovery was made 
by Wollaston ? What are these lines called, and why ? Who first explained 
them ? 480. How can we best observe the lines ? Viewed with a spectroscope, 



THE SPECTKOSCOPE.— EXPEEIMENTS. 265 

several prisms are carefully mounted. We find the spec- 
trum crossed at right angles to its length by numerous 
dark lines — gaps — which we may compare to silent notes 
on an organ. Now, if we light a match and observe its 
spectrum, we find that it is continuous ; it runs from red 
through the whole gamut of color, to the visible limit of 
the violet ; there are no gaps, no dark lines, breaking up 
the band. 

Another experiment. Let us burn something which 
does not burn white ; some of the metals will answer our 
purpose. We see at once by the brilliant colors that fall 
upon our eye from the vivid flame that we have here 
something different. The spectrum, instead of being con- 
tinuous as before, now consists of two or three lines of 
light in different parts ; as if on an organ, instead of press- 
ing down all the keys, we sounded 'but one or two notes in 
the bass, tenor, or treble. 

Let us try still another experiment. We will so arrange 
our prism, that while a sunbeam is decomposed by its 
upper portion, a beam proceeding from burning sodium, 
iron, nickel, copper, or zinc, may be decomposed by the 
lower one. We shall find in each case, that when the 
bright lines of which the spectrum of the metal consists 
flash before our eyes, they will occupy exactly the same 
positions in the lower spectrum as some of the dark bands 
do in the upper solar one. 

481. Here, then, is the germ of Kirchhoff's discovery, 
on which his hypothesis of the physical constitution of the 
Sun is based ; here is the secret of the recent additions to 
our knowledge of the stars, for stars are suns. 

Vapors of metals, and gases, absorb those rays which 
the same vapors of metals' and gases themselves emit. 



what appearances does the solar spectrum present ? Describe the spectrum of a 
match. Describe the spectrum of a substance that does not burn with white 
light— such as some of the metals. Give an account of the third experiment with 
the spectroscope. 481. What principle is at the basis of Kirchhoff's hypothesis ? 
12 



266 THE SPECTRUM. 

482. Facts established by Experiment. — By experiment- 
ing in this manner, the following facts have been estab- 
lished : — 

I. "When solid or liquid bodies are incandescent, they 

give out continuous spectra. 
II. When any gas, or solid or liquid body reduced to the 
state of gas, burns, the spectrum consists of bright 
lines only, and these lines are different for different 
substances. 
III. When light from a solid or liquid passes through a 
gas, the gas absorbs those particular rays of which 
its own spectrum consists. 

483. Fraunhofer's Lines explained. — We now see what 
has become of those rays which the dark lines in the solar 
spectrum tell us are waiting. Before they left the regions 
of our incandescent Sun, they were arrested by those par- 
ticular metallic vapors and gases in his atmosphere with 
which they beat in unison; and the assertion that this 
and that metal exists in a state of vapor in the Sun's 
atmosphere, is based upon their absence. So various and 
constant are the positions of the bright bands in the 
spectra we can observe here, and so entirely do they 
correspond with certain dark bands of the spectrum of the 
Sun, that it has been affirmed that the chances for the cor- 
rectness of the hypothesis are something like 300,000,000 
to 1. 

484. Spectra of the Stars. — Fraunhofer was the first to 
apply this discovery to the stars ; and we have lately reaped 
a rich harvest of facts, in the actual mapping down of the 
spectra of several of the brightest stars, and the examina- 
tion, more or less cursory, of a very large number. In 



482. What three facts as to spectra have been established by experiment ? 483. 
In view of these facts, how are Fraunhofer's lines explained ? What do we con- 
clude from the absence of certain rays in the spectrum ? 484. To what has this 
discovery been extended ? What is found in the case of every star whose spec- 



STELLAR AND NEBULAE SPECTRA. 267 

every case, we find an atmosphere sifting out the rays 
that beat in unison with the metallic and gaseous vapors 
which it contains, and sending to us the residuum, a 
broken spectrum abounding in dark spaces. 

485. Importance of these Researches. — A few words 
will show the great importance of these facts. They tell 
us that, as the solar spectrum contains dark lines, the light 
is due to solid or liquid particles in a state of great heat, 
or incandescence ; and that the light given out by these 
particles is sifted, so to speak, by its atmosphere, which 
consists of the vapors of the substances incandescent in 
the photosphere. Further, as the lines in the reversed 
spectra occupy the same positions as the bright lines given 
out by the glowing particles would do, and as we can by 
experimenting on the different metals match many of the 
lines exactly, we can thus see which light is abstracted, 
and what substance gives out this light. Having done 
this, we know what substances (Art. 126) are burning in 
the Sun. 

Again, we find that all the stars are more or less like 
the Sun, for their spectra exhibit nearly the same appear- 
ances ; we can also tell, as above, what substances are 
burning on their surfaces (Art. 83). 

486. Spectra of the Nebulae. — The spectra of the nebulae, 
instead of resembling that of the Sun and stars, — that is, 
showing a band of color with black lines across it, — con- 
sist of a few bright lines merely. 

487. On August 29th, 1864, Mr. Huggins directed his 
telescope, armed with the spectrum-apparatus, to the plan- 
etary nebula in Draco. At first he suspected that some 
derangement of the instrument had taken place, for no 
spectrum was seen, but only a short line of light, perpen- 



trum is examined ? 485. What conclusions are drawn respecting the Sun from 
investigations of its spectrum ? What do we find with respect to the stars ? 486. 
Describe the spectra of the nebulae. 487. Who examined the spectrum of the 
planetary nebula in Draco ? Give the results of his examination. With what 



268 THE SPECTRUM. 

dicular to the direction of dispersion. He found that the 
light of this nebula, unlike every other celestial light which 
had yet been subjected to prismatic analysis, was not com- 
posed of rays of different refrangibility, as in the case of 
the Sun and stars, and that therefore it could not form a 
spectrum. A great part of the light from this nebula con- 
sists of but one color, and was seen in the spectroscope as 
a bright line. A more careful examination showed another 
line, narrower and much fainter, a little more refrangible 
than the brightest line, and separated from it by a dark 
interval. Beyond this again, at about three times the dis- 
tance of the second line, a third exceedingly faint line was 
seen. 

The strongest line coincides in position with the bright- 
est of the air-lines. This line is due to nitrogen, and oc- 
curs in the solar spectrum about midway between b and/J 
(see Frontispiece). The faintest of the lines of the nebula 
coincides with the line of hydrogen, marked / in the solar 
spectrum. The other bright line was a little less refrangi- 
ble than the strong line of barium. 

488. Here, then, we have three little lines forever dis- 
posing of the notion that all nebulas are clusters of stars. 
With what trumpet-tongue does such a fact speak of the 
resources of modern science ! That nebulae are masses of 
glowing gas is shown by the fact that their light consists 
merely of a few bright lines. 

An object-glass collects a beam of light which would 
otherwise have bathed the Earth forever invisibly to mor- 
tal eye. The beam is passed through a prism, and in a 
moment we find that we have no longer to do with glow- 
ing Suns enveloped in atmospheres enforcing tribute from 
the rays which pass through them, but with something 



does the strongest line of this spectrum coincide ? With what, the faintest line ? 
With what does the other bright line nearly correspond? 488. What important 
fact respecting the physical constitution of nebulae is established by these three 
little lines ? 489. Describe the spectrum of the Moon. What may be inferred 



SPECTEA OF THE MOON AND PLANETS. 269 

devoid of atmosphere, and that something a glowing mass 
of gas (Art. 102). 

489. Spectra of the Moon and Planets. — That moon- 
shine is but sunshine second-hand, and that the Moon has 
no sensible atmosphere, is proved by the fact that in the 
spectroscope there is no difference, except in brilliancy, be- 
tween the two. That the planets have atmospheres is 
shown in like manner, since in their light we find the same 
lines as in the solar spectrum, with the addition of other 
lines due to the absorption of their atmospheres. 

490. Explanation of the Frontispiece. — In the Frontis- 
piece are given representations of the solar spectrum, two 
stellar spectra, the spectra of the Nebula 37, H. iv., and 
the double line of sodium. The latter is shown, to explain 
the coincidences on which our knowledge of the substances 
present in the atmospheres of the Sun and stars depends. 
The light given out by the vapor of sodium consists only 
of the double line shown in the plate. A black double line 
is seen in exactly the same position in the spectra of the 
Sun, Aldebaran, and a Orionis ; hence we infer that sodium 
is present in the atmospheres of all these suns. 

Similarly, were we to observe the spectrum of the vapor 
of iron, in the same position as the 400 or 500 bright bands 
visible in this case, we should see coincident black lines in 
the spectrum of the Sun. The feeble light of the stars 
does not permit all these lines to be observed. It is seen 
in the plate that one of the bright bands in the spectrum 
of the nebula is coincident with one of the lines of nitro- 
gen, and one with the hydrogen line. 

491. In the spectrum of a Orionis, among eighty lines 
observed and measured, no less than five cases of coinci- 
dence have been detected ; that is to say, we have now 
evidence — universally accepted in the analogous case of 

from this ? Describe the spectrum of the planets. What follows ? 490. What 
are represented in the Frontispiece ? Show how we find the substances pres- 
ent in the atmospheres of the stars, by taking sodium and iron as examples. 



270 THE SPECTEUM. 

the Sun — that sodium, magnesium, calcium, iron, and bis- 
muth, are present in the atmosphere of a Orionis. 

492. The Spectroscope. — The Star Spectroscope, with 
which these spectra have been observed, is attached to the 
eye end of an equatorial. As the spectrum of the point 
which the star forms at the focus is a line, the first thing 
done in the arrangement adopted is to turn this line into 
a band, in order that the lines or breaks in the light may- 
be rendered visible. 

The other parts of the arrangement are as follows : — 
A plano-convex cylindrical lens, of about fourteen inches' 
focal length, is placed with its axial direction at right 
angles to the direction of the slit, and at such a distance 
before the slit, within the converging pencils from the ob- 
ject-glass, as to give exactly the necessary breadth to the 
spectrum. Behind the slit, at a distance equal to its focal 
length, is an achromatic lens of 4-J» inches' focal length. 
The dispersing portion of the apparatus consists of two 
prisms of dense flint-glass, each having a refracting angle 
of 60°. 

The spectrum is viewed through a small achromatic 
telescope, provided with proper adjustments, and carried 
about a centre adjusted to the position of the prisms by a 
fine micrometer screw. This measures to about jitot °f 
the interval between A and II oi the solar spectrum. A 
small mirror attached to the instrument receives the light 
which is to be compared directly with the star-spectrum, 
and reflects it upon a small prism placed in front of one 
half of the slit. This light is usually obtained from the 
induction-spark taken between electrodes of different 
metals, raised to incandescence by the passage of an in- 
duced electric current. 

491. What is shown by an examination of the spectrum of a Orionis? 492. To 
what is the star-spectroscope attached ? In the arrangement adopted, what is the 
first thing done ? Describe the other parts of the arrangement. Through what 
is the spectrum viewed? How is the light which is to be compared with the 
star-spectrum received? How is this light iisually obtained ? 493. Describe the 



CELESTIAL PHOTOGKAPHY. 271 

493. A very powerful Spectroscope was for some time 
used at the Kew Observatory, in England, for mapping the 
solar spectrum. The light enters at a narrow slit in one 
of the collimators, which is furnished with an object-glass 
at the end next the prism, to render the rays parallel be- 
fore they enter the prisms. In the passage through the 
prisms the ray is bent into a circle, widening out as it 




494. It is often convenient to use what is termed a 
Direct-vision Spectroscope — that is, one in which the light 

enters and leaves the 
prisms in the same straight 
line. How this is man- 
aged in the Herschel- 

Fig. 102.— Path op the Eat in the Heb- ° * # . 

schel-bbowning Spectboscope. one of the best of its kind, 
by means of successive refractions and reflections, may be 
gathered from Fig. 102. 

495. Celestial Photography. — In both telescopic and 
spectroscopic observations, the visible rays of light are 
used. The chemical rays, however, being also present, 
photographs of the brighter celestial objects can be taken ; 
and celestial photography, in the hands of Mr. De La Rue 
and Mr. Rutherford, has been brought to a high state of 
perfection. The method adopted is to place a sensitive 
plate in the focus of a reflector, or refractor properly cor- 
rected for the actinic rays, and then to enlarge this picture 
to the size required. De La Rue's photographs of the 
Moon, some 1^ inches in diameter, are of such perfection 
that they bear subsequent enlargement to 3 feet. These 
pictures are now being used as a basis of a map of the 
Moon, 200 inches in diameter. 



arrangement for the light in a powerful spectroscope used at Kew for examining 
the solar spectrum. 494. What is it often convenient to use? 495. What is said 
of celestial photography ? What is the method adopted? What use is being 
made of De La Rue's photographs of the Moon ? 



272 UNIVEKSAL GEAVITATION. 

CHAPTER XVI. 
UNIVERSAL GRAVITATION, 

496. Motion. — If a body at rest receive an impulse in 
any direction, it will move in that direction, and with a 
uniform velocity, if it be not stopped. If we set a body 
in motion on the Earth's surface, it will soon be stopped 
by friction. If we fire a cannon-ball in the air, it will in 
time be arrested by the resistance of the air ; moreover, 
while its speed is slackening from this cause, it will fall, 
like every thing else, to the Earth, and its path will be a 
curved line. 

Were it possible to fire a cannon in space where there 
is no air to resist, and were there no body to draw the 
ball to itself, as the Earth does, the projectile would for- 
ever pursue a straight path, with a uniform velocity. As 
it is, the moment the ball leaves the cannon, there is 
superadded to the original velocity of projection an 
acceleration directed toward the Earth; and the path 
described is what is called a resultant of these two 
velocities. 

497. Parallelogram of Forces. — To illustrate resultant 
motion, suppose that the cricket-ball A, in Fig. 103, re- 
ceives an impulse which will send it to B in a certain 
time ; it will move in the direction A B. Suppose, again, 

it receives an impulse 
that will send it to C in 
the same time ; it will 
move in the direction 
< /1 °" B A C\ and more slowlv, 

FlG. 103.-PABALLELOGKAM F FOKCES. ^ -j. ^ & ^ ^^ 

496. If a body at rest receive an impulse in any direction, how will it move? 
What soon stops a body set in motion on the Earth's surface ? By what is a 
cannon-ball fired in the air stopped, and what is its course ? Of what is the path 
described by such a projectile the resultant? 497. Illustrate resultant motion 



TELOCITY OF FALLING BODIES. 273 

to go. But suppose, again, that both these impulses are 
given at the same moment ; it will go neither to JB nor to 
C, but will move in a direction between these points. 
The exact direction, and the distance it will go, are deter- 
mined by completing the parallelogram AJB CD, and 
drawing the diagonal A D, which represents the direction 
and amount of the resultant motion. 

498. Weight. — All bodies left . unsupported fall to the 
Earth ; and it is from this tendency that we derive our 
idea of weight, and of the difference between a light body 
and a heavy one. On the latter point, however, we must 
not allow the action of the atmosphere to mislead us. If 
we drop a dime and a feather, the latter will require more 
time to fall than the former ; it would, therefore, at first 
appear that the tendency to fall, or gravity, of the feather 
is different from that of the dime. This, however, is not 
the case ; for, if we drop them in a long tube, exhausted 
of air, we find that both fall in the same time. The dif- 
ference in the time of falling in the air is due simply to 
the unequal resistance which the air offers to the bodies in 
their descent. 

499. Velocity of Falling Bodies. — Machines have been 
invented for determining the exact rate at which a body 
falls near the Earth's surface. Experiments with these 
show that in the first second it will fall 16^ feet, and 
that the velocity keeps increasing in each subsequent 
second. The following rules have been established : — 

I. To find the space passed through during any second, 
multiply 16^g feet by that one in the series of odd num- 
bers (1, 3, 5, 7, 9, 11, etc.) which corresponds with the 
given second. 

with Fig. 103. 498. Whence do -we derive our idea of weight ? When a dime and 
a feather are dropped, what do we find as regards their respective times of fall- 
ing ? What misapprehension might follow ? How is this proved to he a misap- 
prehension ? Why does the feather take longer to fall than the dime ? 499. What 
do experiments show with respect to the velocity of a falling hody ? Give the 
rule for finding the space passed through during any second. Give the rule for 



274 UNIVERSAL GRAVITATION. 

II. To find the velocity at the termination of any- 
second, multiply lGyL feet by that one in the series of 
even numbers (2, 4, 6, 8, 10, 12, etc.) which corresponds 
with the given second. 

III. To find the whole space passed through in any 
number of seconds, multiply lQ^t ^ eet ^v the square of 
the number denoting the seconds. 

Examples. — What distance will a body fall in the fourth second of its 
descent, what will be its velocity at the end of the fourth second, and 
how far will it have fallen in the first four seconds ? 

1 being the fourth in the series of odd numbers, in the fourth second 
it will fall through 7 times 16^- feet, or 112-&- feet. 

8 being the fourth in the series of even numbers, its velocity at the 
end of the fourth second will be 8 times 16^ feet, or 128| feet, per 
second. 

It will have fallen during the first four seconds 16 (4 2 ) times 16^2 
feet, or 257-j feet. 

500. Curvilinear Motion, how produced. — If a cannon- 
ball were left unsupported at the mouth of a gun, it would 
fall to the Earth in a certain time ; when fired from the 
gun, it has superadded to its tendency to fall a motion 
which carries it to the target. But during its flight 
gravity is constantly at work, and the law referred to in 
Art. 49V holds good in this case also, which is one of 
curvilinear motion. As the cannon-ball is pulled down 
from its straight course toward the target by the action 
of the Earth upon it, so in all cases of curvilinear motion 
there is a something deflecting the moving body from the 
rectilinear course. 

501. Newton's Discovery. — Sir Isaac Newton was the 
first to see that the curved path of the Moon is similar to 
that of a projectile, and that both are due to the same cause 
as the fall of an apple — namely, the attraction of the Earth. 



finding the velocity at the termination of any second. Give the rule for finding 
the whole space passed through in any number of seconds. Illustrate these rules 
with an example. 500. How is curvilinear motion produced ? Show this in the 
case of a cannon-ball. 501. What great discovery was made by Newton ? By 



LAW OF GKAVITY. 275 

He saw that on the Earth's surface the tendency of bodies 
to fall was universal; that the Earth acted, as it were, 
like a magnet, drawing to itself every thing free to move, 
even on the highest mountains ; why not, then, at the dis- 
tance of the Moon ? He immediately applied the knowl- 
edge derived from observations on falling bodies on the 
Earth, to test the correctness of his idea. 

502. Law of Gravity. — Gravity is common to all 
kinds of matter. Its law of action may be stated thus : — 
The force with which two material particles respectively 
attract each other is directly proportional to their masses, 
and inversely proportional to the square of the distances 
between their centres. Now, the intensity of a force is 
measured by the momentum, or joint product of velocity 
and mass, produced in one second in a body subjected to 
its action, — and this measure of force must be remembered 
in discussing the above law of gravity. 

Thus, if our unit of mass be one pound, and if this 
pound be allowed to fall toward the Earth, at the end of 
one second it will be moving with the velocity of (16^ X 2) 
32-J- feet per second. Now let the mass be a ten-pound 
weight ; it might be thought that, since the Earth attracts 
each pound of this weight, and therefore attracts the 
whole with ten times the force with which it attracts one 
pound* we should have a much greater velocity produced. 
The old schoolmen thought so ; but Galileo showed that a 
ten-pound weight will fall to the ground with the same 
velocity as a one-pound weight. This fact is quite con- 
sistent with our definitions of gravity and force. Un- 
doubtedly the ten-pound weight is attracted with ten 
times the force, — but then there is ten times the mass to 
move; so that, although the velocity produced in one 
second is no greater than in the case of the one-pound 

what reasoning did he arrive at this conclusion ? 502. Is gravity confined to any- 
particular kind of matter? State the law of gravity. By what is the intensity 
of a force measured? Illustrate this in the case of a one-pound and a ten-pound 



276 UNIVEESAL GEAVITATION. 

weight, yet if we multiply this velocity by the mass the 
momentum produced is ten times as great. 

503. Now, since each individual atom of the Earth 
attracts each individual atom of the weight, we might 
expect, from our definition of gravity, as well as from the 
well-known law that every action has a reaction, that the 
Earth, when the weight is dropped, at the end of one 
second rises toward the weight with the same momentum 
that the weight falls to the Earth. No doubt it does; 
but as the Earth is a very large mass, this momentum 
represents a velocity infinitesimally small. 

504. Effect of an Increase of Mass in the Attracting 
Body. — It follows from the above law that, if the mass 
of the Earth were twice as great as it now is, it would 
produce in a falling body twice the present velocity, or 
64^- feet per second ; and were it only half as great, we 
should have but half the present velocity produced, or 
16^3- feet per second. 

Accordingly, at the surface of the Moon the force of 
gravity is very small, whereas on the Sun it is enormous. 
A man carried to the Moon and retaining the same 
muscular power, could jump six times as high as on the 
Earth's surface ; whereas, if carried to the Sun, he would 
be so strongly attracted by its immense mass that he 
would be literally crushed by his own weight. 

505. Effect of Distance.— A body at the surface of the 
Earth, or 4,000 miles from its centre, acquires, as we have 
seen, by virtue of the Earth's attraction, a velocity of 32-J- 
feet per second at the end of the first second. During 
this second, however, it has not fallen 321 f ee t ; for, as it 
started from a state of rest, and acquired the velocity of 

weight, and show that the velocity in both ca^es is the same. 503. What move- 
ment might we expect in the Earth, when a weight is dropped ? Does the Earth 
move toward the weight ? With what velocity, and why ? 504. What is the effect 
of an increase, and what of a decrease, of mass ? What facts are stated with re- 
spect to the force of gravity at the surface of the Moon and the Sun ? 505. How 
far does a falling body descend in the first second, near the Earth's surface? 



ACTION OF GEAVITY ON THE MOON'S PATH. 277 

32-J. feet only at the end, it will have gone through the 
first second with the mean velocity of 16 T ^- feet, and will, 
in fact, have fallen only that distance. Now, this body, at 
the distance of the Moon, or sixty times as far from the 
Earth's centre as it now is, would fall in one second toward 
the Earth only ^^ of 16^ feet. Let us see how we 
know this. 

506. The Moon' 3 orbit is an exact representation of 
what the path of our cannon-ball would be at the Moon's 
distance from the Earth. In fact, 
the Moon's path MJV, in Fig. 104, 
is the result of an original impulse 
in the direction MB, at right angles 
to EM, and a constant attraction 
toward the Earth — the amount of 
attraction being represented for the 
arc MJV, by the line MA. To 
find the value of MA. let us take 

Fig. 104.— Action op Gravity , •»*- 

on the Moon's Path. the arc described by the Moon in 

one minute, the length of which is found by the following 

proportion to be nearly 33" : — 

27d. 7h. 43m. : lm. : : 360° : arc described in lrn. 

The arc MJV, then, being 33" for one minute of time, 
the length of MA can be readily calculated ; it is found 
to be 16^- feet when ME equals 240,000 miles. That is, 
a body at the Moon's distance falls as far in one minute 
as it would do on the Earth's surface in one second ; in 
one second, therefore, by Rule III. Art. 499 (as 60s. make 
lm., and 60 2 = 3600), it will fall but -^g^o of the distance 
it would fall in one second at the Earth's surface. 

Now, the Moon, being 240,000 miles from the Earth's 
centre, is just 60 times as far from it as an object at the 



How far would it fall in the same time at the Moon's surface? 506. Give the 
reasoning by which this fact is arrived at. How, as far as distance is concerned, 
is the force of gravity thus experimentally found to vary ? What did this calcu- 




278 UNIVEKSAL GKAVITATION. 

Earth's surface is, and we have seen that it is affected by 
the Earth's attraction only -g-^-g- as much. Its distance is 
60 times greater, its gravity is 60 2 times less. Thus the 
force of attraction is experimentally found to vary in- 
versely as the square of the distance. It was this calcula- 
tion that revealed to Newton the law of universal gravita- 
tion. 

507. Kepler's Laws. — Long before Newton's discovery, 
Kepler, from observations of the planets merely, had 
detected certain laws of their motion, which bear his 
name. They are as follows : — 

I. Each planet describes round the Sun an elliptical 
orbit, and the centre of the Sun occupies one of 
the foci. 
II. The radius-vector of a planet describes equal areas 

in equal times. 
III. If the square of the time of revolution of each 
planet be divided by the cube of its mean dis- 
tance from the Sun, the quotient will be the same 
for all the planets. 

508. Kepler's Second Law. — It was stated in Art. 301 
that the planets move faster as they approach the Sun. 
Kepler's second law enables us to find how much faster. 

The Radius-vector of a planet is the line joining the 
planet and the Sun. If the planet described a circle, the 
radius-vector would always be of the same length ; but in 
elliptical orbits its length varies, and the shorter it be- 
comes, the more rapidly does the planet move. 

509. In Fig. 105 are shown the orbit of a planet with 
its eccentricity exaggerated, and the Sun situated in one 
of the foci. The three shaded areas are equal, — the part 
of the orbit intercepted being shortest where the radius- vec- 

lation reveal to Newton ? 507. What is meant by Kepler's Laws ? Give Kepler's 
three laws. 508. What was stated in Art. 301 ? What does Kepler's second law 
enable us to find ? What is the Radius-vector of a planet ? Tn what kind of or- 
bits does the radius-vector vary, and how ? 509. Explain the second law, with 
Fig. 105. Show from the figure how the velocity at perihelion and aphelion must 



KEPLEK'S LAWS. 



279 




Fig. 105.— Illustration of Kepler's Second Law. 



tor is longest, 
as must be the 
case in order to 
make the areas 
equal. 

The arcs 
AB, CD, and 
EF, are re- 
sp e c t ively 
those described 
at perihelion, 
at aphelion, 
and at mean 
distance, and according to the second law they are trav- 
ersed in equal times. Therefore, as a greater distance has 
to be got over at perihelion, and a less one at aphelion, 
than when the planet is at its mean distance, the motion 
in the former case must be more rapid, and in the latter 
case slower, than in other parts of the orbit. 

510. Kepler's Third Law. — The third law shows that 
the periodic time of a planet and its distance from the 
Sun are in some way related ; so that, if we represent the 
Earth's distance and periodic time by 1, and know the, 
period of any planet in terms of the Earth's period, we 
can at once determine its distance from the Sun in terms 
of the Earth's distance, by a simple proportion. Thus, in 
the case of Jupiter : — 



Square of 
Earth's 
period 



Square of 

Jupiter's 

period 



Cube of 
Earth's 
distance 



Cube of 
Jupiter's 
distance 



1x1 



11.86X11.86 



1X1X1 



140.559 



That is, whatever the distance of the Earth from the Sun 
may be, the distance of Jupiter is 4^ 140 times greater. 



compare with that at mean distance. 510. What does Kepler's third law show 
us ? What must we know, to determine a planet's distance from the Sun in terms 
of the Earth's distance? Illustrate this in the case of Jupiter. 511. What does 



280 



UNIVEESAL GKAVITATION. 



511. The following table 
we are considering : — 



shows the truth of the law 





Periodic Time. *§£££? f 


lime square 

divided by 

distance cube 


Mercury . 


87.97 . 


0.3871 . 


133,421 


Venus 


224.70 . 


0.7233 . 


133,413 


Earth 


365.25 . 


1.0000 . 


133,408 


Mars . . 


686.98 . 


1.5237 . 


133,410 


Jupiter 


4332.58 . 


. 5.2028 . 


133,294 


Saturn 


10759.22 . 


. 9.5388 . 


133,401 


Uranus 


30686.82 . 


19.1824 . 


133,422 


Neptune . 


60126.72 . 


30.0368 . 


133,405 



512. Centrifugal and Centripetal Force. — As these laws 
were given to the world by Kepler, they simply represented 
facts, but Newton showed that they all established the 
truth of the law of gravitation and flowed naturally from 
it. He proved that the motion of a planet in any part of 
its orbit is the result of two forces — one the original 
impulse, which gives it a tendency to move off from its 
orbit in a tangent, and which is called the Centrifugal 
Force — the other the attraction of the Sun, which deflects 
it toward that body, and is called the Centripetal Force. 

513. Newton also showed that 'the attraction is pro- 
portional to the product of the masses of the bodies. 
That if we take two bodies, the Sun and our Earth, for 
instance, we may imagine all the gravitating energies of 
each to be concentrated at its centre; and that, if the 
smaller one receives an impulse neither exactly toward 
nor from the larger, it will describe an orbit round the 



the table show ? How do you find the results to agree ? In the case of what 
planet is there the greatest deviation ? 512. What did Newton show with respect 
to these laws of Kepler ? Of what did he prove that the motion of a planet in 
any part of its orbit is the result? 513. To what did Newton show that the at- 
traction is proportional ? Where may we imagine all gravitating energy to be 
concentrated ? What did Newton show with regard to the smaller of two 
bodies bound together by mutual attraction ? What kind of an orbit will it de- 
scribe ? What would follow, if the attraction of the central body were to cease ? 



CENTEIFUGAL AND CENTEIPETAL FOECE. 



281 




Fig. 306.— The Conic Sections 



larger. That this orbit will be 
one of the conic sections — that 
is, either a circle, ellipse, hyper- 
bola, or parabola (see Fig. 106). 
Which of these it will be, de- 
pends in each case on the direc- 
tion and force of the original 
impulse, which, since the move- 
ments of the heavenly bodies 
are not arrested as bodies in 
motion on the Earth's surface 
are, is still at work. 

Were the attraction of the 

AB, circle; CD, ellipse; EF, central body to Cease, the re- 
hyperbola; GH, parabola. -■ . , -, in , 

volvmg body would leave its 
orbit, in consequence of the centrifugal tendency it ac- 
quired at its start; were the centrifugal tendency to 
cease, the centripetal force would be uncontrolled, and 
the body would fall upon the attracting mass. 

514. We may now inquire how it is that, according to 
Kepler's second law, equal areas are traversed in equal 
times. 

The direction of a body moving round another in a 
circular orbit is always at right angles to the line joining 

the two bodies. If the 
orbit be elliptical, the 
direction is thus per- 
pendicular only at two 
points ; i. e., at the apsi- 
des, or extremities of the 
major axis — the aphelion 

Era. 107.-Vaeying Velocity of a Body and perihelion points. 

MOVING IN AN ELLIPTICAL ObBIT, EX- ^ . r 

plained. In Fig. 1 07, the planet 




What, if the centrifugal tendency were to cease ? 514. What is always the direc- 
tion of a body revolving round another in a circle? If the orbit be elliptical, 
where alone is the direction thus perpendicular ? With Fig. 107, explain the 



282 UNIVERSAL GRAVITATION. 

JP is moving in the direction Pf, the tangent to the 
ellipse at the place it occupies ; this direction not being 
at right angles to the radius-vector, the attractive force 
of the Sun helps the planet along. At B it is evident that 
the attractive force is pulling the planet back. At Q the 
centripetal force is strong, but the planet is enabled to 
overcome it by the increased centrifugal tendency it has 
acquired from being acted on at JP. At "Fthe centripetal 
force is weak, but the planet is not able to overcome it, 
on account of its centrifugal tendency's having been 
diminished from being acted on as at JR. 

515. Gravity not dependent on the Mass of the Attracted 
Body. — We learned in Article 498 that the attraction 
which a body exerts is the same on all bodies equally dis- 
tant from it, without reference to their mass. A dime and 
a feather are equally attracted by the Earth. In like man- 
ner, if we had the Sun, Jupiter, a pea, and a mass twice as 
great as the Sun, at the same distance from the Earth, the 
Earth's attraction would draw them all through the same 
number of feet in a second. 

516. Centre of Gravity and Motion. — Since the amount 
of attraction is proportioned to the mass of the attracting 
body (Art. 502), it follows that the attraction of a body 
with 1 unit of mass will be 1,000 times less than that of a 
body with 1,000 units of mass — this proportion being, of 
course, kept up at all distances. If in the case of two 
bodies, such as the Earth and Sun, all the attractive force 
were confined, say, to the Sun, then the Earth would re- 
volve round the Sun, the Sun's centre being the centre of 
motion. But as the Earth draws the Sun, as well as the 
Sun the Earth, both Earth and Sun revolve round a point 
in a line joining the two, called the Centre of Gravity. 



varying velocity of a body moving in an elliptical orbit. 515. Of wbat is tbe at- 
traction which a body exerts entirely independent? 516. If all the attractive 
force were confined to one of two bodies, what would be the result ? As they 
mutually attract each other, what follows ? What is meant by the Centre of 




CENTRE OF GRAVITY. 283 

5 1 7. The centre of gravity and motion would be deter- 
jl \g mined, if we could join the 

|jp £ ill) two bodies by a bar, and 

Y TT find the point of the bar at 

Fig. ios.- Centre of Gravity and Mo- which (supported on a ful- 

tion in the case or Equal Masses. crum ) they would balance 

each other. It is clear that, if the two bodies were of equal 
mass, this point would be half-way between the two, as at 
C in Fig. 108. If one were 
heavier than the other, the 
point of support would ap- 
proach the heavier body in 

the ratio of its greater Fig. 109.— Centre of Gravity and Mo- 

weight (see Fig. 109). In TI0N IN THE CASE 0F Unequal Masses ' 
the case of the Sun and Earth, for instance, the centre of 
gravity of the two lies within the Sun's surface. 

518. Determination of Masses. — It follows, from what 
has been seated, that the masses of the Sun, and of those 
planets which have satellites, can be determined, if the mass 
of our own Earth and the distances of the attracted bodies 
from their centres of motion are known. Since, for in- 
stance, the planets revolve round the Sun, from the curva- 
ture of their paths, we can determine the amount of the 
Sun's attraction, — which, it will be remembered, is pro- 
portioned directly to his mass, and is wholly independent 
of the mass of the attracted body. "Having ascertained 
the Sun's attractive force, and adjusted it to the distance 
of 4,000 miles from his centre, we can compare it with that 
of the Earth, and find how many times greater his mass is 
than the Earth's (Art. 522). In like manner, we can weigh 
Jupiter, Saturn, Uranus, and Neptune, by finding the effect 
their attraction has on the orbits of their satellites — also 

Gravity? 517. How could the centre of gravity and motion be determined? 
Where would it lie, if the bodies were of equal mass ? Where, if one were heavier 
than the other ? Where does it lie in the case of the Sun and the Earth ? 518. 
How can the masses of the Sun and of those planets that have satellites be deter- 
mined ? How can the masses of those double stars whose distances are known 



284 



UNIVEKSAL GEAVITATION. 



the double stars whose distances are known, by measuring 
their effect on each other's orbit. 

519. Determination of the Earth's Density and Mass. — 
We must first find the Earth's mass, or weight. It is not 
sufficient to determine its bulk, because it might be light, 
like a gas, or heavy, like lead. The mean density, or 
specific gravity, of its materials — that is, how much they 
weigh, bulk for bulk, compared with some well-known sub- 
stance, such as water — must be determined. 

520. The Earth's density has been determined in three 
ways : — 

I. By comparing the attractive force of a large metallic 
ball of known size and density, with that of the Earth. 

II. By finding how much a large mountain will deflect 
a plumb-line, or draw it toward itself from the perpendic- 
ular. 

III. By determining the rate of vibration of the same 
pendulum on the top and at the bottom of a mountain, or 

at the bottom of a mine 
and at the Earth's sur- 
face. 

C21. The Cavendish 
Experiment. — It will 
here suffice to describe 
the first-mentioned meth- 
od, adopted by Caven- 
dish in 1798. The weight 
of any thing is a measure 
of the Earth's attrac- 
tion. Cavendish, there- 

Fig. 110.— The Cavendish Experiment. A B, r. ■, , ■.-, 

the small leaden balls on the rod G. D E, lore > tooK tw0 Small 

the suspending wire. F G, the large leaden leaden balls of known 
balls on one side of the small ones. HE, , 

the large leaden balls in a position on the Weight, and fixed them 

otherside " at the ends of a slender 



(H 




be determined ? 519. What must we first find? Why is it not sufficient to. de- 
termine the Earth's bulk ? 520. In what three ways has the Earth's density been 



EARTH'S MASS, HOW DETERMINED. 285 

wooden rod six feet long, suspended by a fine wire. When 
the rod was at rest, he placed two large leaden balls one 
on either side of the small ones. If the large balls exerted 
any appreciable attractive influence on the smaller ones, 
the wire would twist to allow each small ball to approach 
the large one near it; and a telescope was arranged to 
mark the deviation. 

Cavendish found there was a deviation. This enabled 
him to calculate how great it would have been had each 
large ball been of the size of the Earth. He then had the 
attraction of the Earth (measured by the weight of the small 
balls), and the attraction of a mass of lead as large as the 
Earth, as the result of his experiment. The density of the 
Earth, then, was to the density Of lead as the attraction of 
the Earth was to the attractive force of a leaden ball as large 
as the Earth. This proportion gave the Earth a density 
of 5.45 as compared with water, the density of lead being 
11.35. With this density, the mass of the whole Earth 
can readily be determined ; it amounts in round numbers 
to 

6,000,000,000,000,000,000,000 tons. 
But this number is not needed in Astronomy; the relative 
masses indicated in Art. 157 are sufficient. 

522. Determination of the Sun's Mass. — We are now 
prepared to determine the Sun's mass, if we can find how 
many times it is greater than that of the Earth. This we 
can do by comparing the action of the Sun and the Earth 
on a falling body. 

On the Earth's surface, i. e., 4,000 miles from its centre, 
a body falls 16^ feet in a second. Can we determine how 
far it would fall at 4,000 miles from the centre of the Sun ? 
This is easy : by the process used in the case of the Moon 
(Art. 506), we find that the Earth falls to the Sun .0099 
feet in a second. But this is at a distance of 91,000,000 

determined ? 521. Give an account of the Cavendish experiment. What was the 
Earth's density thus found to be? What is its mass ? 522. Give the details of 



286 . UNIVERSAL GRAVITATION. 

miles from the Sun's centre. We must bring this to 4,000 
miles from the Sun's centre, or 22,750 times nearer, — which 
we do by multiplying the square of 22,750 by .0099, since 
attraction varies inversely as the square of the distance. 
The result is 5,123,758 feet. Then 

ft. ft. 

16 i • 512*™ •• 1 • i the Sun's mass in 
lb^ . 5,123,753 . . 1 . -j termg Qf the Eartll , g# 

Solving this proportion, we find that the Sun's mass is 
approximately 318,641 times greater than that of the Earth. 
The exact figures are given in Table IV. of the Appendix. 

523. Similarly, from the orbit of any of the satellites 
we determine its rate of fall at 4,000 miles from the centre 
of its primary, and by the same process as above we find 
the mass of its primary in terms of the Earth's mass. 

5 24. Determination of the Comparative Force of Gravity. 
— The force of gravity on the surface of the Sun or a planet, 
compared with that on our Earth, may be determined in the 
following manner : — 

Let us take the case of the Sun. Tf we express the 
Sun's mass . and radius in terms of the Earth's, then the 
force of gravity on the Sun's surface, in terms of that on 
the Earth's surface, will be 

Sim's mass 314760 _ 

Square of radius ~~ 107.8 2 ~~ 27 * 

525. Perturbations. — We have seen that it is the at- 
traction of gravitation which causes the planets and satel- 
lites to pursue their paths round the central body ; that 
their motion is similar to that of a projectile fired on the 
Earth's surface, if we leave out of consideration the resist- 
ance of the air ; and that Newton's law enables us to de- 
termine the masses of the Sun and of the other central 



the process by which the Sun's mass is determined. What is it found to be in 
terms of the Earth's mass ? 523. By the same process, what further may we de- 
termine ? 524. How is the force of gravity on the Sun's surface found ? 525. 
What have we found with respect to the motion of the planets and satellites ? 



PERTURBATIONS AND INEQUALITIES. 287 

bodies from the motions of the bodies revolving round 
them, when the mass of the Earth itself is known. 

Now, the orbit which each body would describe round 
the Sun or round its primary, if itself and the Sun or pri- 
mary were the only bodies in the system, is liable to varia- 
tions in consequence of the attractions of the other planets 
and satellites. These irregular attractions, which vary ac- 
cording to the constantly-changing distances between the 
bodies, are called Perturbations, and the resulting changes 
in the motions of the bodies affected are called Inequalities 
if the disturbances are large, and Secular Inequalities if 
they extend over a long period of time. 

526. These perturbations and inequalities are among 
the most difficult subjects in the whole domain of astrono- 
my ; it is sufficient here to say that it is by carefully ob- 
serving them that we have been able to determine the 
masses of the planets that have no satellites and of the 
satellites themselves. 

527. We shall conclude with an explanation of two 
additional and very important effects of attraction of a 
somewhat different kind. One results from the attractions 
of the Sun and Moon on the equatorial protuberance of the 
Earth, and is called the Precession of the Equinoxes ; the 
other is due to the attraction of the water on the Earth's 
surface by the Sun and Moon, whence result the Tides. 

528. Precession of the Equinoxes, how produced. — Let 
the equatorial protuberance of the Earth be represented 
by a ring, supported by two points at the extremities of 
a diameter, and inclined to its support as the Earth's 
equator is inclined to the ecliptic. Let a long string be 
attached to the highest portion of the ring, and be pulled 
horizontally, at right angles to the line connecting the 

What does Newton's law enable us to do ? What is meant by Perturbations ? 
What are Inequalities ? What are Secular Inequalities ? 526. By carefully ob- 
serving these inequalities, what have we been enabled to determine ? 527. What 
two effects of attraction remain to be considered ? From what does each result ? 
528. Illustrate the effect of the Sun's attraction in producing precession, with a 



288 UNIVEESAL GEAVITATION. 

two points of suspension, and away from the centre of the 
ring. This pull will represent the Sun's attraction on the 
protuberance. The effect on the ring will be that it will 
at once take a horizontal position; the highest part of 
the ring will fall as if it were pulled from below, the low- 
est part will rise as if pulled from above. 

The Sun's attraction on the equatorial protuberance in 
certain parts of the orbit is exactly similar to the action 
of the string on the ring, but the problem is complicated 
by the two motions of the Earth. In the first place, in 
consequence of the yearly motion, the protuberance is pre- 
sented to the Sun differently at different times, so that 
twice a year (at the solstices) his action is greatest, and 
twice a year (at the equinoxes) it is reduced to 0. In the 
second place, the Earth's rotation is constantly varying 
that part of the equator subjected to the attraction. 

5 29. If the Earth were at rest, the equatorial protuber- 
ance would soon settle down into the plane of the eclip- 
tic ; in consequence, however, of its two motions, this re- 
sult is prevented, and the attraction of the Sun on a 
particle situated in the protuberance is limited to causing 
that particle to meet the plane of the ecliptic earlier than 
it otherwise would do. If we look at the Earth as pre- 
sented to the Sun at the winter solstice (Fig. 44), and 
bear in mind that the Earth's rotation is from left to right 
in the diagram, it will be clear, that, while the particle is 
mounting the equator, the Sun's attraction is pulling it 
down ; so that the path of the particle is really less steep 
than the equator is represented in the diagram. Toward 
the east, the particle descends from this less height more 
rapidly than it would otherwise do, as the Sun's attraction 



ring and a string. By what is the prohlem complicated ? What is the conse- 
quence of the yearly motion ? What is the consequence of the Earth's rotation ? 
529. If the Earth were at rest, what would the equatorial protuberance soon do ? 
What is the effect of the Earth's two motions ? If we look at the Earth as pre- 
sented to the Sun at the winter solstice in Fig. 44, what will appear ? What does 



PEECESSION OF THE EQUINOXES. 289 

is still exercised. The final result therefore is, that it 
meets the plane of the ecliptic sooner than it would other- 
wise have done. 

What happens with one particle in the protuberance 
happens with all. One half of it, therefore, tends to fall, the 
other half to rise ; and the whole Earth meets the strain 
by rolling on its axis. The inclination of the protuber- 
ance to the plane of the ecliptic is not altered ; but, in con- 
sequence of the rolling motion, the places in which it 
crosses that plane precede those at which the equator 
would cross it were the Earth a perfect sphere ; hence the 
term precession. 

530. In the above explanation, no mention has been 
made of the sphere enclosed in the equatorial protuber- 
ance, as the action of the Sun on the spherical portion is 
constant. It plays an important part, however, in aver- 
aging the precessional motion of the entire planet during 
the year, acting as a brake at the solstices, when the 
Sun's effect on the equatorial protuberance is greatest, and 
continuing the motion at the equinoxes, when, as before 
stated, the Sun's action is reduced to 0. 

We have also, for the sake of greater clearness, left 
the Moon out of view, although our satellite plays the 
greatest part in precession, for the following reason. The 
action referred to does not depend on the actual attrac- 
tions of the Sun and Moon upon the Earth as a whole, 
which are in the proportion of 190 to 1, but on the differ- 
ent degrees of attraction exerted by each upon different 
parts of the Earth. As the Sun's distance is so great 
compared with the diameter of the Earth, the differential 
effect of the Sun's action is small ; but, as the Moon is so 
near, her differential effect and consequent influence in 
producing precession is three times that of the Sun. 

one-half of the protuberance tend to do, and what the other ? How does the Earth 
meet the strain ? What is the consequence of the rolling motion ? 530. What is 
the effect of the sphere enclosed in the equatorial protuberance ? What part does 
13 



290 UNIVERSAL GRAVITATION. 

531. Change in the Earth's Axis. — The change in the 
position of the equator which follows from the rolling 
motion, is necessarily connected with a change in the 
Earth's axis. This change consists in a slow revolution 
round the axis of the celestial sphere, perpendicular to the 
plane of the ecliptic. 

532. Nutation. — Superadded to the general effect of 
the Sun and Moon in causing the precession of the' equi- 
noxes, is an effect due to the Moon alone, termed Nuta- 
tion. 

The Moon's nodes perform a complete revolution in 
nineteen years. Consequently, for half this period the 
Moon's orbit is less inclined to the plane of the Earth's 
equator than the ecliptic is ; during the other half, 
the orbit is inclined so that its divergence from the 
plane of the Earth's equator is the greatest possible. 
In the former position the precessional effect will be 
small, while in the latter it will be the greatest possi- 
ble. 

Were the pole of the earth at rest, nutation would 
cause it to describe a small ellipse every nineteen years. 
Since, however, the pole is in motion, as we just saw in 



Fig. UI.-Path of the Pole op the Equator, P, round the Pole op the 
Heavens (or Ecliptic), Q. 

Art. 531, the two motions are compounded, so that the 
path of the pole of the equator round the pole of the eclip- 



the Moon perform in producing precession ? Why is its effect greater than that 
of the Sun ? 531 What motion is produced in the Earth's axis, in consequence 
of the change in the position of the equator ? 532. What effect, due to the Moon 
alone, helps to produce the precession of the equinoxes? What is meant by 
Nutation? If the pole of the Earth were at rest, what would nutation cause it 



TIDES. 291 

tic, instead of being circular, is icaved, as shown in Fig. 

111. 

533. The effect of this motion of the Earth's axis on the 
apparent position of the heavenly bodies, and the correc- 
tions which are thereby rendered necessary, have already 
been referred to in Art. 459. 

534. Tides— Tides are alternate risings and fallings of 
the waters of the ocean. 

The waters gradually rise for about six hours, forming 
flood tide,— remain stationary a few moments at high tide, 
—then begin to fall, forming ebb tide,— reaching low tide 
in about six hours ; and then, after a few minutes' rest, 
these movements are repeated. The interval between two 
successive high or low tides is 12 hours 27 minutes,— that 
is, they rise and fall twice in a lunar day. 

535. Spring and Neap Tides.— We not only have two 
tides in a lunar day, but twice in the lunar month— from one 
and a half to two days after new and full Moon— the tides 
are higher than usual : these are the Spring Tides. Twice 
also, after the Moon is in her quadratures, they are lower 
than usual : these are the Neap Tides. It will be gathered 
from the foregoing that the tides have something to do 
with the Moon. In fact, these phenomena are due to the 
attraction of the Sun and Moon on the fluid envelope of 
the Earth — not to their absolute, but (as in the case of 
precession) to their differential, action ; and the two periods 
correspond with the lunar day and the lunar month, be*- 
cause the Moon's differential attraction is about three 
times as great as that of the Sun. 

536. Tides, how produced.— It may be stated, then, 
generally, that the semi- diurnal tides are caused by the 
Moon (although there is really a smaller daily tide caused 

to do ? Since it is in motion from another cause, what is the consequence ? 534. 
What are Tides ? Describe the succession of tides. What is the interval be- 
tween two successive high or low tides ? Why is it just 12 hours and 2? minutes ? 
535. What is meant by Spring Tides and Neap Tides ? To what are tides due ? 
Why are they specially connected with the lunar day and month ? 536. How are 



292 UNIVEESAL GKAVITATION. 

by the Sun) ; and that the semi-monthly variation in their 
height is due to the Sun's tide being added to that of the 
Moon when she is new and full, at which time the Sun and 
Moon pull together, — and subtracted from it at the first 
and last quarters, when they are pulling at right angles to 
each other. 

The spring and neap tides thus produced are also 
affected by the difference of latitude between the two 
bodies. Of course, that spring tide will be highest which 
occurs when the Moon is nearest her node, or in the eclip- 
tic. The apex of the semi-diurnal tide, also, follows the 
Moon throughout her various declinations. 

537. The double daily tide arises from the action of 
the Moon on both the water and the Earth itself. On 
the 'side toward the Moon, the water is pulled from the 
Earth and piled up under the Moon, as the Moon's action 
on the surface-water is greater than its action on the 
Earth's centre, which is more remote. In like manner, 
since the Moon's attraction for the Earth's centre is greater 
than its attraction for the water on the opposite side of 
the Earth, and since the solid Earth must move with its 
centre, the Earth is pulled from the water. Hence there 
are always two tides on the Earth's surface; and this 
double tide is simply a state of the water, without pro- 
gressive motion and nearly at rest under the Moon. There 
is, in fact, an ellipsoid of water enclosing the Earth, which 
always remains with its longer axis pointing to the Moon. 

538.' The high water under, or nearly under, the Moon, 
is not caused merely by the direct attraction of our satel- 
lite acting upon the particles immediately beneath it, but 
by its action on all the particles of water on the side of 
the Earth turned to it,, all of which tend to close up under 



ordinary tides, and the spring and neap tides, produced ? By what are the spring 
and neap tides also affected ? 537. Explain why there is a double daily tide. 538. 
What "besides the direct attraction of the Moon on the particles immediately be- 
neath it helps to produce the tides ? What is meant hy the tangential component 



TIDE£. 293 

the Moon. The force acting upon these particles is called 
the tangential component of the attraction ; and this is 
by far the most powerful cause of the tides, as it acts at 
right angles to the Earth's gravity, whereas the direct at- 
traction of the Moon acts in opposition to this gravity. 

539. Establishment of the Port— The phenomena of the 
tides are greatly complicated by the irregular distribution 
of land. The time of high water at any one place occurs 
at the same period from the Moon's passage over the me- 
ridian ; this period is different for different places. The in- 
terval at new or full Moon between the time of the Moon's 
meridian passage and high water is termed the Establish- 
ment of the Port. 

540. Velocity and Height of the Tidal Wave-In the 
open ocean, the velocity of the tidal wave may be as great 
as 900 miles an hour ; in shallow waters, it may be retarded 
to even 7 miles, while its height may be greatly increased. 
The average height of the tide round the islands in the 
Atlantic and Pacific Oceans is but 3 J feet ; whereas at 
the head of the Bay of Fundy it is 70 feet. 

54.1. Effect of Tidal Action on the Daily Rotation.— As 
the tidal wave, being regulated by the Moon, does not 
move so rapidly as the Earth, it appears to move westward 
while the Earth is moving eastward ; and it has been sug- 
gested that this movement acts as a brake on the Earth's 
daily rotation, causing a constant but very slight de- 
crease in its velocity. The apparent acceleration of the 
Moon's mean motion may be accounted for by supposing 
that the sidereal day is shortening, in consequence of tidal 
action, at the rate of -^ of a second in 2,500 years. 

of the attraction ? 539. By what are the phenomena of the tides greatly com- 
plicated? What is meant by the Establishment of the Port? 540. How great 
may the velocity of the tidal Avave be in the open ocean ? What is it sometimes 
in shallow waters ? What is the average height of the tide round the islands in 
the Atlantic and Pacific ? At the head of the Bay of Fundy ? 541. What is the 
effect of tidal action on the Earth's daily rotation ? How much may we suppose 
the sidereal day to be shortened in consequence of tidal action ? 



APPENDIX 



Table I. 
EXPLANATION OF ASTRONOMICAL SYMBOLS. 

Signs of the Zodiac. 



0. T Aries . . . 





VI. '*L Libra . . . . 180 


I. b Taurus . . . 


. 30 


VII. m Scorpio ... 210 


II. n Gemini . . . 


. CO 


VIII. t Sagittarius . . 240 


III. © Cancer . . . 


. 1)0 


IX. \3 Capricornus . . 270 


IV. Si Leo ... . 


. 120 


X. Xw' Aquarius . . . 300 


Y. ttjZ. Virgo . . . 


. 150 


XI. X Pisces .... 330 


The Sun . . . 


. O A Comet . . . . f 


The Moon . . . 


. $ A Star -x- 


<5 Conjunction. 




° Degrees. 


□ Quadrature. 




' Minutes of Arc. 


8 Opposition. 




" Seconds of Arc. 


Q Ascending Node. 




R.A., or AV.., or a., Right As- 


y Descending Node. 




cension. 


h. Hours. 




Dec 1 -, D., or d, Declination. 


m. Minutes of Time. 




N. P. D., North-polar 


s. Seconds of Time. 




Distance. 


Greek Alph 


abet, used in naming the Stars. 


a Alpha. 


i Iota. p Eho. 


P Beta. 


k Kappa. a Sigma. 


y Gamma. 


A Lambda. r Tau. 


d Delta. 


/« Mu. v Upsilon. 


e Epsilon. 


v Nu. ^ Phi. 


C Zeta. 


1 Xi. * Chi. 


V Eta. 


o Omicron. i> Psi. 


Theta. 


7T Pi. to Omega. 




Major Planets. 


S Mercury. 




% Jupiter. 


? Venus. 




*> Saturn. 


or J The Eartl 


i. 


J£I Uranus. 


£ Mars. 




f Neptune. 



APPENDIX. 



295 



ASTEROIDS, OR MINOR PLANETS. 



© Ceres. 

Pallas. 

® Juno. 

Vesta. 

Astraea. 

Hebe. 

Iris. 

Plora. 

Metis. 

® Hygeia. 

@ Parthenope. 

© Victoria. 

© Egeria. 

© Irene. 

© Eunomia. 

© Psyche. 

© Thetis. 

© Melpomene. 
@ Portuna. 
© Massilia. 
© Lutetia. 
© Calliope. 
© Thalia. 
© Themis. 
© Phocea. 
© Proserpine. 
© Euterpe. 
© Bellona. 
© Amphitrite. 
© Urania. 
© Euphrosyne. 
© Pomona. 
© Polyhymnia. 
© Circe. 
© Leucothea. 
@ Atalanta. 
© Pides. 



© Leda. 

© Laetitia. 

© Harmonia. 

© Daphne. 

© Isis. 

© Ariadne. 

© Nysa. 

© Eugenia. 

© Hestia. 

© Aglaia. 

© Doris. 

© Pales. 

© Virginia. 

© Nemausa. 
© Europa, 
© Calypso. 
@ Alexandra. 
© Pandora. 
© Melete. 
© Mnemosyne. 
© Concordia. 
© Olympia. 
© Echo. 
© Danae. 
© Erato. 
© Ausonia. 
@ Angelina. 
© Maximiliana. 
© Maia. 
© Asia, 
(g) Leto. 
© Hesperia. 
© Panopea. 
© Mobe. 
© Feronia. 
© Clytie. 
© Galatea. 



© Eurydice. 
© Freia. 
© Frigga. 
© Diana. 
© Eurynomc. 
© Sappho. 
© Terpsichore. 
© Alcmene. 
© Beatrix. 
© Clio. 
© Io. 
© Semele. 
© Sylvia. 
Thisbe. 
© Julia. 
© Antiope, ' 
© iEgina. 
© Undina. 
© Minerva. 
© Aurora, 
© Arethusa. 
© JEgle. 
© Clotho. 
© Ianthe. 
© Dike. 
© Hecate. 
© Helena. 
© Miriam. 
© Hera. 
© Clymene. 
© Artemis. 
© Dione. 
© Camilla. 
© Hecuba. 
© Felicitas. 
© Lydia. 
© Ate: etc. 



296 



APPENDIX. 



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APPENDIX. 



297 



§1 

*5 


\ Galileo. 

Sir W. Ilerschel. 
Cassini. 

a 

Huyghens. 
Bond and Lassell. 
Cassini. 

Lassell. 

0. Struve. 

Sir W. Ilerschel. 

Lassell. 


Appar- 
ent 
Star 
Magni- 
tude. 


i>. .t- O .t- 1>IOMCSIOQ0 1>Ci cv. k. tv.. cc. "* 
r-l i— 1 i— I i— ( i — 1 . i — 1 *- .*- >- ,_| 


1 

5 


Miles. 
2,153 

2,252 
2,099 
3,436 
3,057 

1,000 

500 

500 

1,200 

3,300 

1,800 


Inclination of 
Orbit to 
Plane of 
Ecliptic. 


5 8 40 

3 4 6 
3 5 5 
3 9 2 
3 28 

►23 10 22 

100° 34' 
Ascending 
" Node 
165° 30' 

157? 


' ■* ' x 


"3-2 

si 


27 7 43 

1 18 27 

3 13 14 

7 3 43 
16 16 32 

22 37 

1 8 53 

1 21 18 

2 17 41 

4 12 25 
15 22 41 
21 7 8 
79 7 55 

2 12 28 

4 3 27 

8 16 55 
13 11 6 

5 21 8 


lag* 

5 £ 


Miles. 
237,640 

267,380 

425,160 

678,390 

1,192,820 

120,800 
155,000 
191,000 
246,000 
343,000 
798,000 
1,007,000 
2,314,000 

123,000 
171,000 
281,000 
376,000 

220,000 


Name of 
Satellite. 


Moon. 

Io. 

Europa. 

Ganymede. 

Calhsto. 

Mimas. 

Enceladus. 

Tethys. 

Dione. 

Ehea. 

Titan. 

Hyperion. 

Japetus. 

Ariel. 
Umbriel. 
Titania. 
Oberon. 


o 


rH rH CM CO -# H « M ^ IO CO i- CO rH CM CO "^ rH 


a 


Earth. 
Jupiter. 

Saturn. 

Uranus 

[Motion of 

Satellites 

retrograde]. 

Neptune 

[Motion possibly 
• retrograde]. 



298 APPENDIX. 

Table IV.— THE SUN. 

Old Value. New Value. 

Equatorial Horizontal Parallax .... 8.5776" 8.940" 

Mean Distance from the Earth . . . 95,274,000 91,430,000 

Diameter in miles 888,646 852,584 

Inclination of Axis to Plane of Ecliptic . . 82° 45' ) ~ 10 ~ rt 
-r -x -. « ^ i y. > for 18o0 

Longitude of Node 73 40 



Mass 

Density .... 

Volume ..... 

Force of Gravity at | 

Equator .... J 



Earth's tak- 
en as 1. 



f 354,930 314,760 

0.250 0.250 

1,415,225 1,245,126 

28.7 27.2 



Time of Eotation Variable with the Latitude. 

Apparent Diameter as seen from the Earth : — 

Maximum 32' 36.41" 

Minimum 31' 32" 

Mean 32' 4.205" 



Table V. 

ADDITIONAL ELEMENTS OF THE MOON. 

Mean Horizontal Parallax 57' 2.70" 

Mean Angular Telescopic Semi-diameter . . 15 34.4 

Ascending Node of Orbit 13° 53' 17" 

Mean Synodic Period 29.530588715 d - 

Time of Eotation 27.321661418 d -. 

Inclination of Equator to Plane of Ecliptic . . 1° 30' 10.8" 

Longitude of Pole ? 

Daily Geocentric Motion 13° 10' 35" 

Mean Eevolution of Nodes 6793.39108 d - 

Mean Eevolution of Apogee or Apsides . . . 3232.57343 d- 

Density, Earth's as 1 0.56654 

Volume, " 0.02012 

Force of Gravity at surface, Earth's as 1 . . ^ 

JBodies fall in one second 2.6 feet. 



APPENDIX. 



299 



Table VI.— TIME. 



I. THE YEAE. 



The Mean Sidereal Year . . . 
The Mean Solar or Tropical Year 
The Mean Anomalistic Year . . 



Mean Solar Days, 
d. h. m. s 

365 6 9 9. 



365 
365 



II. THE MONTH. 

Lunar or Synodic Month 29 

Tropical Month 27 

Sidereal " 27 

Anomalistic " 27 

Nodical " 27 



12 
7 
7 

13 

5 



48 
13 



44 
43 
43 
18 
5 



46.054440 
49.3 



2.84 

4.71 

11.54 

37.40 

35.60 



III. — THE DAT. 

The Apparent Solar Day, or interval between 

two transits of the Sun over the meridian . variable. 

The Mean Solar Day, or interval between two 

transits of the Mean Sun over the meridian 24 

The Sidereal Day 23 56 4.09 

The Mean Lunar Day 24 54 



Table VII.— CORRECTION" FOE REFRACTION". 



Apparent 
Altitude. 


Mean 
Refraction. 


Apparent 
Altitude. 


Mean 
Refraction. 


Apparent 
Altitude. 


Mean 
Eefraction. 


o / 


/ // 


o / 


/ // 


o / 


/ // 





34 54 


7 


7 20 


25 


2 3 


20 


30 52 


8 


6 30 


30 


1 40 


40 


27 23 


9 


5 49 


35 


1 22 


1 


24 25 


10 


5 16 


40 


1 9 


2 


18 9 


11 


4 49 


45 


58 


3 


14 15 


•12 


4 25 


50 


48 


3 30 


12 48 


13 


4 5 


60 


33 


4 


11 39 


14 


3 47 


70 


21 


5 


9 47 


15 


3 32 


80 


10 


6 


8 23 


20 


2 37 


90 






ALPHABETICAL INDEX 



ETYMOLOGICAL VOCABULARY OF ASTRONOMICAL TERMS. 



[The numbers refer to Articles, not to Pages ] 



Abbreviations used in Astronomy, 
see Appendix, Table I. 

Aberration of light (ab, from, and 
errare, to wander), 410 ; results of, 
452 ; correction for, 452 ; constant 
of, 452 ; spherical and chromatic, of 
lenses, 425, 426. 

Absorption of the atmospheres of 
stars, 84; of Sun, 124. 

Acceleration, Secular, of the Moon's 
mean motion, an increase in its ve- 
locity caused by a slow change in the 
eccentricity of the Earth's orbit, 541. 

Achromatism (a, without, and 
xpw/ota, color) of lenses, 424. 

Adams, discovered Neptune, 141. 

Aerolites (ii?p, the air, and \i9o$, a 
stone), meteoric stones which fall to 
the Earth's surface, 317. 

Aerosiderites (afc and o-tSrjpo?, 
iron), pieces of meteoric iron which 
fall to the Earth's surface, 317. 

Aerosiderolites, (ob?p, o-iS^jpos, and 
Aiflos), pieces of meteoric iron and 
stone which fall to the Earth's sur- 
face, 317. 

Aigrettes, rays of light seen through 
the corona in a total eclipse of the 
Sun, 249. 

Air, refraction of the, 411. 

Algol, the variable star, 75. 

Almanac, Nautical, 463. 

Alphabet, Greek, 59. 



Altazimuth (contraction of altitude 
and azimuth), an instrument for 
measuring altitudes and azimuths, 
442; when used, 441. 

Altitude (altitudo, height), the angu- 
lar height of a celestial body above 
the horizon, 332. 

Anaximander, conceived the idea 
of a plurality of worlds, 21. 

Angle (angulus, a corner), the differ- 
ence in direction of two straight 
lines that meet, 26 ; how named, 26 ; 
right, obtuse, and acute, defined, 27 ; 
measurement of angles, 438; of posi- 
tion ' (439), the angle formed by the 
line joining the components of double 
stars, with the direction of the diur- 
nal motion. It is reckoned in de- 
grees from the north point passing 
through east, south, and west. 

Angle of the vertical, the difference 
between astronomical and geodetical 
latitude. It is at the equator and 
at the poles, and attains a maximum 
ofll'30"inlat. 45°. 

Annular (annulus, a ring), eclipses, 
244; nebulae, 95. 

Anomalistic, month, 399, 400 ; year, 
401, 402. 

Anomaly (<*> not, and ojuaAds, equal). 
The anomaly is either true, mean, or 
eccentric. The first is the true dis- 
tance of a planet or comet from peri- 



ALPHABETICAL INDEX. 



301 



helion ; the second, what it would 
have been had it moved with a mean 
velocity ; and the third, an auxiliary 
angle introduced to facilitate the 
computation of a planet's or comet's 
motion. 

Ansae (Lat. handles) of Saturn's rings, 
282. 

Anti-trades, 207-209. 

Aphelion (anb, from, and rjXto?, the 
Sun), the point in an orbit farthest 
from the Sun, 175 ; distance of com- 
ets, 298 ; of planets, see Appendix, 
Table II. ; change of, 407. 

Apogee (<i7rb and y?i, the Earth). (1) 
The point in the Moon's orbit far- 
thest from the Earth, 218. (2) The 
position in which the Sun or other 
body is farthest from the Earth. 

Apsis (<xi|hs, a curve), plural Apsides. 
The line of apsides (407) is the line 
joining the aphelion and perihelion 
points ; it is therefore the major axis 
of elliptic orbits. 

Arabians, the chief astronomers of 
the Dark Ages, 22. 

Aratus, described the leading con- 
stellations in verse, 54. 

Arc, Diurnal, the path described by a 
heavenly body between rising and 
setting, 360; Semi-diurnal, half this 
path on either side of the meridian. 

Arcturus, proper motion of, 63. 

Aries, First Point of, one of the points 
of intersection of the celestial equator 
and ecliptic, and the starting-point 
for It. A. and celestial longitude, 
328. 

Aristarchus, taught that the planets 
revolve round the Sun, 21. 

Aristotle, his opinion respecting the 
Milky Way, 46. 

Ascending- Node, 221. 

Ascension, Right, the angular dis- 
tance of a heavenly body from the 
first point of Aries, measured on the 
equator, 328. 

Aspects of the planets, 370. 

Asteroids (acmyp, a star, and elSos, 
form), minor planets, 137 ; discovery 
of, 142, 291 ; size, 292 ; force of grav- 
ity, 292 ; orbits, 293 ; evidences of at- 
mosphere and rotation, 294 ; mode 
of discovery, 295 ; theory respecting, 
296. 

Astronomy (ao-Typ, a star, and v6p.o$, 



a law), defined, 1 ; usefulness of, 14 : 
early history of, 19-23. 

Atmosphere (ar^os, vapor, and <r4>al- 
pa, a sphere), of stars, 82 ; of Sun, 
124, 126 ; of Earth, 205-211, 214 ; of 
Moon, 233; of Mars, 270; of Jupiter, 
277 ; refraction of the, 411. 

Attraction of gravitation, see Gravi- 
tation. 

Axis, the line on which a heavenly 
body rotates : the major axis of an 
elliptical orbit is the line of apsides ; 
the minor. axis is the line at right 
angles to it; the semi-axis major is 
equal to the mean distance. 

Axis of the Earth, its inclination, 180 ; 
motion of, 531 ; inclination of Sun's, 
112. 

Axis, polar, and declination, of equa- 
torials, 436. 

Azimuth (samatha, Arabic, to go 
toward), the angular distance of a 
celestial object from the north or 
south point of the meridian, 332. 



Baily's Beads, notched appearance 
sometimes presented in the narrow 
ring of light in an annular eclipse, 
250. 

Base-line, 469. 

Bayer, John, his method of naming 
the stars, 58. 

Belts, of Jupiter, 276 ; of calms and 
winds, 207; of Saturn, 281. 

Biela's Comet, 303. 

Bissextile {Ms, twice, and sextus, 
sixth), 403. 

Bode's Law, 290. 

Bolides, 307. 

Bond, his discoveries respecting Sat- 
urn's rings, 282. 

Brilliancy, of the stars, 41 ; Sun, 
106 ; Moon, 224 ; minor planets, 295. 



Calendar, 403-405. 

Calms, equatorial, 207 ; of Cancer and 
Capricorn, 207 ; polar, 207. 

Catalogues of stars, 449. 

Cavendish experiment, the, 521. 

Celestial, sphere, 326-329; apparent 
movements of, 334 ; two methods of 
dividing, 357 ; meridian, 333. 

Centre (tcivrpov) of gravity, 516, 517. 

Centrifugal Force, 512, 513. 



302 



ALPHABETICAL INDEX. 



Centripetal Force, 512, 513. 

Chaldeans, their early progress in 
Astionomy, 20. 

Chinese, their early attention to As- 
tronomy, 20. 

Chronograph (xpovos, time, and 
ypafyio, I write), an instrument for 
determining the time of transit of a 
heavenly body across the field of 
view of a transit-circle or other in- 
strument, 447. 

Circle, defined, 31 ; great and small, 
defined, 38 ; declination, the circle 
on the declination axis of an equato- 
rial, by which the declinations of 
celestial bodies are measured, 436 ; 
transit, an instrument for observing 
the transit of heavenly bodies across 
the meridian and their zenith-dis- 
tance, 443; of perpetual apparition, a 
circle of polar distance equal to the 
latitude of the place, the stars within 
which never set, 339. 

Circumference of a circle, defined, 
31 ; how divided, 32. 

Circumpolar stars, 338. 

Clepsydrae (/<Aei//v8pa, a water-clock), 
384. 

Clock, principles of construction, 388 ; 
invention of, 389 ; Tycho Brahe's 
regulator, 389 ; application of the 
pendulum, 389 ; sidereal, 397. 

Clock-stars, stars the positions of 
which have been accurately deter- 
mined, used in regulating astronomi- 
cal clocks and determining the time. 

Clouds, on the Earth, 211 ; on Mars, 
270. 

Clusters of stars, 87, 88. 

Co-latitude of a place or a star is the 
difference between its latitude and 
90°. 

Collima'ion (cum, with, and limes, a 
limit), line of the optical axis of a 
telescope ; error of, the distance of 
the cross-wires of a telescope from 
the line of collimation, 438. 

Collimator, a telescope used for de- 
termining the line of collimation in 
fixed astronomical instruments. 

Colors of stars, 78-80. 

Colures (koAovw, I divide), meridians 
passing through the equinoxes and sol- 
stitial points, called the equinoctial and 
solstitial colures. 

Coma (Lat. hair) of a comet, 300. 



Comes (Lat. companion), the smaller 
component of a double star. 

Comets (KO/U.1JT7J5, long-haired), prob- 
ably masses of gas, 13, 304 ; orbits of, 
298; distances from Sun, 298, 299; 
long and short period comets, 298 ; 
head, nucleus, and tail, 300; changes 
as they approach the Sun, 301 ; en- 
velopes, 301; velocity of, 301 ; prob- 
ably harmless, 302; division of Bie- 
la's comet, 303 ; physical constitu- 
tion of, 304 ; numbers of, in our sys- 
tem, 305 ; how formerly regarded, 
300. 

Compression, polar, the amount by 
which the polar diameter of a planet 
is less than its equatorial one; of the 
Earth, 202, 203 ; of Mercury, 260 ; of 
Venus, 264; of Jupiter, 273. 

Cone of shadow in eclipses, 244. 

Conic sections, the, 513. 

Conjunction. Two or more bodies 
are said to be in conjunction when 
they are in the same longitude or 
right ascension. In inferior con- 
junction, the bodies are on the same 
side of the Sun ; in superior conjunc- 
tion, on opposite sides, 370. 

Constant of aberration, 452. 

Constellation (cam, with, and stella, 
a star), a group of stars supposed to 
represent some figure, 53; by whom 
arranged, 54 ; zodiacal, 55 ; northern, 
56 ; southern, 57 ; circumpolar, 342, 
343; visible on different evenings 
throughout the year, 349-351. 

Copernicus, discovered the true sys- 
tem of the universe, 22. 

Copernicus, lunar crater, 230. 

Corona (Lat. crotvn), the halo of light 
which surrounds the dark body of 
the Moon during a total eclipse of 
the Sun, 249. 

Corrections applied to observed 
places, 450-460; for refraction, 451; 
aberration, 452; parallax, 454 ; pre- 
cession of the equinoxes and nuta- 
tion, 456-4G0. 

Corrugations, on the Sun's disk, 
120. 

Cosmical rising and setting of a 
heavenly body, its rising or setting 
with the Sun. 

Craters (/cparTjp, a mixing-bowl) of 
the Moon, 227-230. 

Crust of the Earth, 193-197 ; tempera- 



ALPHABETICAL INDEX. 



303 



ture of, 199- thickness, 200 ; density, 
201. 

Culmination {culmen, the top), the 
passage of a heavenly hody across 
the meridian, when it is at the high- 
est point of its diurnal path. 

Curtate distance, the distance of a 
celestial body from the Sun or Earth 
projected on the plane of the ecliptic. 

Cusp (cuspis, a sharp point), the ex- 
tremities of the illuminated side of 
the Moon or inferior planets at the 
crescent phase. 

Cycle (kuxAos, a circle) of eclipses, 247. 



Dawes, his description of the red- 
flames seen during the total eclipse 
of the Sun in 1860, 251 ; discovery of 
Saturn's inner ring, 282. 

Day, apparent and mean solar, 395 
civil, 395 ; sidereal and solar, 353 
and night, how produced, 182, 183 
length of, in different latitudes, 184 
how to find the length of. 361. 

Declination, the angular distance of 
a celestial body north or south from 
the equator, 328; parallels of, 328; 
axis of equatorials, 436. 

Degree, the 360th part of any circle, 
32. 

De La Rue, Mr., his lunar, solar, and 
planetary photographs, 495. 

Democritus, his opinion respecting 
the Milky Way, 46. 

Density, what it is, 155; of the Earth, 
156 ; how determined, 520 ; of the 
Earth's crust, 201 ; of the Sun, 109 ; 
of the planets, 157. 

Descending- Node, 221. 

Detonating meteors, 316. 

Diameter, of the Earth, 162; Moon, 
217 ; Sun, 108 ; planets, 150 ; of heav- 
enly bodies, how found, 475. 

Digit, the twelfth part of the diam- 
eter of the Sun or Moon, used in 
measuring the extent of a partial 
eclipse, 246. 

Dimensions, of the Sun, 108 ; Earth, 
162 ; Moon, 217 ; lunar craters, 227 ; 
the planets, 150 ; Saturn and his 
rings, 284 ; how determined, 475. 

Diodorus, his opinion respecting the 
Milky Way, 46. 

Disk (SiVkos), the visible surface of the 
Sun, Moon, or planets, 105. 



Dispersion of light, 414; varies in 
different substances, 415. 

Distance, of stars, 43, 44 ; how deter- 
mined, 474 ; of nebulae, 99 ; of Sun, 
107; how determined, 472; old and 
new value of, 472; of planets from 
Sun, 148; how determined, 469; of 
asteroids, 293 ; of Moon from Earth, 
218 ; polar, 329 ; how distances are 
measured, 469. 

Donati's Comet, 300. 

Double Stars, 66, 68. 



Earth, the. a planet, 11 ; is round, 
160; rotation proved by Foucault, 
163, 164; proved by the gyroscope, 
165, 166; poles, 162; equator, 162; 
diameters, 162 ; parallels and me- 
ridians, 167 ; latitude, 168 ; longitude, 
169 ; tropics, polar circles, and zones, 
170; length of polar and equatorial 
diameter, 171; shape, 171, 172, 202, 
203 ; motions of, 173 ; effects of mo- 
tions, 181 ; shape of orbit, 174, 176 ; 
change in, 406 ; eccentricity of orbit, 
176 ; when in perihelion, 176 ; veloci- 
ty of rotation, 177 ; velocity of revo- 
lution, 178; inclination of axis, 180 ; 
day and night, 182 ; how caused, 183 ; 
length of, 184 ; how to determine, 
361 ; seasons, 186-189 ; structure and 
past history, 192-197 ; crust, of what 
composed. 193 ; interior temperature 
of, 198, 199 ; once a star, 197; density 
of the crust, 201; atmosphere, 205- 
211 ; belts of calms and winds, 207 ; 
cause of winds, 210 ; elements in the 
Earth's crust, 212, 213 ; in the Earth's 
atmosphere, 214 ; appearance, as seen 
from Moon, 218. Apparent move- 
ments.— The Earth, the centre of the 
visible creation, 324 ; apparent move- 
ments of the heavens, due to the real 
movements of ihe, 325 ; effects of ro- 
tation, 325, 344 ; apparent move- 
ments of the stars, as seen from dif- 
ferent points on the surface, 335-337 ; 
effects of the Earth's yearly motion, 
345, 346 ; Earth's way, 453; effects of 
attraction of. 501 ; motion of axis, 531. 

Earth-shine, 223. 

Eccentricity (ex, from, and centrum, 
a centre), of an ellipse, the distance 
of either focus from the eentre, di- 
vided by half the major axis, 35. 



304 



ALPHABETICAL INDEX. 



Eclipses (e/cAeu/as, a disappearance), 
237-255 ; explanation of, 238 ; of the 
Moon, 241, 242; of the Sun, 243-245; 
annular, explained, 244 ; recurrence 
of, 247 ; phenomena attending a total 
eclipse of the Sun, 248 ; number of, 
253; memorable, 254; effects on the 
ignorant, 255. 

Ecliptic (so called because, when 
either Sun or Moon is eclipsed, it is 
in this circle), the great circle of the 
heavens, along which the Sun per- 
forms his annual journey, 358 ; plane 
of the, 111" 145 ; secular variation of 
the obliquity of the ecliptic, 458. 

Egress, the passing of one body off 
the disk of another; e. g., one of the 
satellites off Jupiter, or Venus off the 
Sun. 

Egyptians, their early progress in 
Astronomy, 20. 

Elements, chemical, present in the 
Sun, 126 ; in fixed stars, 83 ; in the 
Earth's crust, 212 ; in meteorites, 
321. 

Elements of an orbit, quantities the 
determination of which enables us to 
know the form and position of the 
orbit of a comet or planet, and to 
• predict the positions of the body, see 
Appendix, Tables II.-V. 

Ellipse, defined, 35; how it may be 
drawn, 35 ; major and minor axis, 
defined, 35 ; different forms of, 174 ; 
one of the conic sections, 513. 

Elongation, the angular distance of 
a planet from the Sun, 372 ; of Mer- 
cury and Venus, 372. 

Emersion, the reappearance of a 
body after it has been eclipsed or 
occulted by another; e. g., the emer- 
sion of Jupiter's satellites from be- 
hind Jupiter, or of a star from be- 
hind the Moon. 

Encke's Comet, 298, 299, 301, 304. 

Envelopes of Comets, 301. 

Ephemeris (em, for, and -q/xepa, a 
day), a statement of the positions 
of the heavenly bodies for every day 
or hour, prepared some time before- 
hand, 463. 

Epoch, some common period for 
which the positions of the heavenly 
bodies are calculated, 460. 

Equation of the centre, the differ- 
ence between the true and the mean 



anomaly of a planet or comet ; of the 
equinoxes, the difference between the 
mean and apparent equinox ; of time, 
the difference between true solar and 
mean solar time, 394. 

Equator, of a sphere, 38 ; terrestrial, 
162 ; celestial, 327. 

Equinoctial Line, see Equator. 

Equatorial, telescope, 436 ; method 
of using, 448; horizontal parallax, 
455. 

Equinoxes (cequus, equal, and nox, 
night), the points of intersection of 
the ecliptic and equator, 183; the 

• Earth as seen from the Sun at the, 
189 ; precession of the, 457 ; how pro- 
duced, 528-530. 

Eratosthenes, measured the Earth's 
circumference, 21. 

Establishment of the port, 539. 

Evection (evehere, to carry away). 
One of the moon's inequalities which 
increases or diminishes her mean 
longitude to the extent of 1° 20'. 

Evening Star, 263. 

Eye-pieces of telescopes, 431 ; tran- 
sit eye-piece, 446. 



Faculee (Lat. torches), the brightest 
parts of the solar photosphere, 119. 

Palling Bodies, velocity of, 499. 

Field of view, the portion of the heav- 
ens visible in a telescope. 

Fixed stars, see Stars. 

Flora, the asteroid nearest the Sun, 
293. 

Focus (Lat. hearth), the point at 
which converging rays meet, 418. 

Foci of an eclipse, 35. 

Forces, parallelogram of, 497. 

Fossils (Lat.fossilis, dug), 195. 

Foucault, proves the Earth's rota- 
tion, 163, 164 ; determines the velo- 
city of light, 410. 

Fraunhofer's Lines, 479, 483. 



Galaxy (yd\aKTos, of milk), the Greek 
name for the Milky Way, or Via Lac- 
tea, 46. 

Galileo, first used the telescope, 23, 
427; construction of his telescope, 
431. 

Geocentric (v>), the Earth, and *eV- 
rpov, a centre), as viewed from the 



ALPHABETICAL INDEX. 



305 



centre of the Earth; latitude and 
longitude, 355. 

Gibbous (Lat. gibbus, bunched) Moon, 
236. 

Globes, use of the, 340; celestial, 61 ; 
rectifying the globe, 340, 347 ; globe, 
celestial, explains Sun's daily mo- 
tion, 359, 360. 

Gnomon (yvu>,u.<ov, an index), a sun- 
dial, 385-387. 

Granules on the solar surface, 120. 

Gravitation, Universal, 502 ; the 
Moon's path, 506; Kepler's laws, 
507; results of, 525; perturbations, 
525; precession, 528; nutation, 532; 
tides, 534, 536. 

Gravity (gravis, heavy), 498; meas- 
ure of, on the Earth, 499 ; on the Sun 
and planets, 524 ; centre of, 506, 517. 

Great Bear, the constellation, 53, 
342. 

Greeks, their additions to astronomi- 
cal knowledge, 21. 

Gregorian calendar, 403. 

Gyroscope (yvpos, a circle, and <r/co- 
ireta, I see), 166. 



Halley's Comet, 301. 

Harvest Moon, 363. 

Head of comets, 300. 

Heavens, how to observe the, 347- 
349. 

Heliacal rising or setting of a star is 
when it just becomes visible in even- 
ing, or invisible in morning, twilight. 

Heliocentric (t?a.ios, the Sun, and 
nevrpov, a centre), as seen from, or 
referred to, the centre of the Sun ; 
latitude and longitude, 355. 

Heliometer (rjAios, and fxirpov, a 
measure), a telescope with a divided 
object-glass, designed to measure 
small angular distances with great 
accuracy ; so called because first 
used to measure the Sun. 

Hemispheres Cq/u-i, half, and o-fyalpa, 
a sphere), half the surface of the ce- 
lestial sphere. 

Herschel, Sir "W., discovered the 
inner satellites of Saturn, 282; dis- 
covered Uranus, 140; principle of his 
reflector, 434. 

Hindoos, their ideas of an eclipse, 
255. 

Hipparchus, catalogued the stars, 21. 



Horizon (6pi£w, I bound), true or ra- 
tional, 330 ; sensible, 161, 330. 

Horizontal Parallax, 455. 

Hour-angle, the angular distance of 
a heavenly body from the meridian. 

Hour-circle, the circle attached to 
the equatorial telescope, by which 
right ascensions are indicated, 448. 

Hug-gins, Mr., his spectroscopic ob- 
servations, 487. 

Huyghens, discovered the true na- 
ture of Saturn's rings, 282 ; his ar- 
rangement of lenses in eye-pieces. 
431. 

Hyperbola (vTrep^oAT/), one of the 
conic sections, 513. 



Immersion (immergere, to plunge in- 
to), the disappearance of one heav- 
enly body behind another, or in the 
shadow of another. 

Inclination of an orbit, the angle 
between the plane of the orbit and 
the plane of the ecliptic ; of the Sun, 
112; of the Earth, 180 ; of the axis of 
the planets, see Appendix, Table II. 

Inequalities, Secular; perturbations 
of the celestial bodies so small that 
they become important only in a long 
period of time, 525. 

Inferior Conjunction, 370. 

Inferior Planets, 256. 

Instruments, astronomical, 427-448. 

Irradiation, 223. 



Jets in comets, 301. 

Jovicentric (Jovis, of Jupiter, and 
nevrpov, a centre), as seen from, or 
referred to, the centre of Jupiter. 
j Julian calendar, 403. 

Jupiter, 273 ; distance from the Sun 
and period of revolution, 148 ; diam- 
eter, 150 ; volume, mass, and density, 
157 ; polar compression, 273 ; sea- 
sons, 274 ; description of, 275, 276 ; 
belts of, 276 ; its rapid rotation, 276 ; 
probably surrounded by an immense 
atmosphere, 277 ; satellites, 278, 279. 



Kepler's Laws, 507; second law, 
explained, 508, 514 ; third law, illus- 
trated and proA r ed, 510, 511. 

Kirch.h.off's investigations of spec- 
tra, 479, 481. 



306 



ALPHABETICAL INDEX. 



Latitude {latitudo, breadth), terres- 
trial, 168 ; how obtained, 465 ; celes- 
tial, 355 ; how obtained, 461 ; latitude 
of a place is equal to the altitude of 
the pole, 339 ; Geocentric, Heliocen- 
tric, Jovicentric, Saturnicentric, lati- 
tude as reckoned from the centre of 
the Earth, Sun, Jupiter, and Saturn. 

Liens, what it is, 416 ; its action on 
a ray of light 416 ; kinds of lenses, 
417 ; refraction by convex, 418 ; re- 
fraction by concave, 423 ; axis of a, 
420; achromatic, 424; chromatic and 
spherical aberration of, 426. 

Le Verrier, discovered Neptune, 141. 

Librations of the Moon, 220. 

Light, what it is, 408 ; velocity of, 15, 
409 ; aberration of, 410 ; refraction 
and reflection, 411^13; dispersion, 
414. 

Limb, the edge of the disk of the 
Moon, Sun, or a planet. 

Line, defined, 24 ; straight and curved, 
defined, 25; parallel lines, defined, 
25; line of apsides, 407; of nodes, 
the imaginary line between the as- 
cending and descending node of an 
orbit, 221. 

Longitude (longitudo, length), terres- 
trial, 169; how determined at sea, 
191, 468 ; in fixed observatories, 467 ; 
celestial, 355 ; how determined, 461 ; 
mean, the angular distance from the 
first point of Aries of a planet or 
comet supposed to move with a mean 
rate of motion ; Geocentric, Helio- 
centric, Jovicentric, Saturnicentric, 
longitude as reckoned from the cen- 
tre of the Earth, the Sun, Jupiter, 
and Saturn. 

Lunar Distances, used to deter- 
mine terrestrial longitudes, 468. 

Lunation (lunatio), the period of the 
Moon's journey round the Earth, 399. 

Luni-solar precession, see Preces- 
sion. 



Magellanic clouds, 47. 

Magnitudes of stars, 40, 41. 

Major axis, see Axis. 

Mars, 266; distance from the Sun 
and period of revolution, 148 ; diam- 
eter, 150 ; volume, mass, and density, 
157 ; day and year, 266 ; description 
of, 267 ; how presented to the Earth 



in different parts of its orbit, 269, 
379-381 ; its land, water, and clouds, 
270 ; its ice and snow, 271 ; seasons, 
272 ; how its distance from the Earth 
is determined, 471. 

Mass, the quantity of matter a heav- 
enly body contains ; of Sun, 109 ; how 
determined, 522 ; of planets, 157 ; how 
determined, 523, 526. 

Maximiliana, the asteroid farthest 
from the Sun, 293. 

Mean distance of a planet, etc., is half 
the sum of the aphelion and perihe- 
lion distances. This is equal to the 
semi-axis major of an elliptic orbit, 
148. Mean anomaly, see Anomaly ; 
mean obliquity is the obliquity un- 
affected by nutation ; mean time, 
395 ; mean Sun, 393. 

Mercury, 257 ; distance from Sun and 
period of revolution, 148 ; phases, 
257; orbit and apparent diameter, 
258 ; heat and light, 259 ; seasons, 
259 ; day and year, 260 ; density and 
force of gravity on its surface, 260 ; 
polar compression, 260 ; mountains, 
261 ; distance from the Sun, 148 ; 
diameter, 150 ; relative volume, mass, 
and density, 157 ; elongation, 372. 

Meridian (meridies, midday), the 
great circle of the heavens passing 
through the zenith of any place and 
the poles of the celestial sphere, 167. 

Metals and metalloids, list of, 212; 
present in the Sun and stars, 10. 

Meteorites, 317; how divided, 317; 
remarkable meteoric falls, 319 ; chem- 
ical composition of, 320, 321 ; struc- 
ture of, 322. 

Meteors (luerewpov), luminous, their 
position in the system, 138 ; divi- 
sions of, 307; numbers seen in a 
star-shower, 307 ; explanation of star- 
showers, 308-311 ; the November 
ring, 308 ; radiant-point, 311 ; cause 
of brilliancy, 313 ; shape of orbits, 
312 : weight of, 314 ; velocity of, 313; 
August and April showers, 315; de- 
tonating meteors, 316 ; sporadic, 
318. 

Metius, invented the telescope, 427. 

Micrometer (juiKpos, small, and ju.e- 
rpov, measure), an instrument with 
fine movable wires attached to eye- 
pieces, to measure small angular dis- 
tances, 439. 



ALPHABETICAL INDEX. 



307 



Microscopes (micpos, small, and o-ko- 
ttc'w, I see), of the transit-circle, 446. 

Milky Way, 46 ; opinions of the 
ancients respecting it, 46 ; stars in- 
crease in number as they approach 
it, 48 ; nebulae do not, 101. 

Minor, planets, 137 ; axis, see Axis. 

Mira, "the marvellous," 74. 

Month, the, 399 ; length of the lunar 
and other months, 400. 

Moon, why its shape changes, 12; 
size, 217; distance, 218; orbit, 218, 
222 ; period of revolution, 219 ; libra- 
tions, 220 ; nodes, 221, 247 ; Moon's 
path concave with respect to the Sun, 
222; earth-shine, 223; brightness of 
Moon, 224; apparent difference of 
size, 225 ; description of surface, 
227-231; no water or atmosphere, 
232; rotation, 234; day, 234; phases, 
235, 236 ; eclipses, 241, 242 ; apparent 
motions, 362 ; harvest Moon, 363 ; 
action of gravity on path of, 506 ; in- 
fluence of. in producing precession, 
530; produces nutation, 532; influ- 
ence of, in producing tides, 536-538 : 
elements of, see Appendix, Table V. 

Moons, see Satellites. 

Morning Star, 263. 

Motion, proper, of stars, 63 ; appar- 
ent, of planets, 364-374 ; direct and 
retrograde, 373; laws of, 496, 497; 
resultant, 497 ; curvilinear, how pro- 
duced, 500. 

Mountains, lunar, heights of, 229. 

Nadir (natara, to correspond), the 
point beneatb the feet, 327. 

Neap tides, 535. 

Nebulee (nebula, a cloud), why so 
called, 7 ; are masses of gas, 13, 102, 
488 ; classification of, 93 ; light of, 99 ; 
variability of, 100; distribution cf, 
101 ; physical distribution of, 102 ; 
spectrum analysis of, 486. 

Nebular hypothesis, 103, 216. 

Nebulous stars, 98. 

Neptune, 289 ; distance from the Sun 
and period of revolution, 148 ; diam- 
eter, 150 ; volume, mass, and density, 
157; discovery of, 141; light, heat, 
and density, 289. 

Newton, discovered the law of gravi- 
tation, 23, 501. 

Night, succession of day and, 182 ; 
bow to find the length of, 361. 



Nodes (nodus, a knot), the points at 
wbich a comet's or planet's orbit in- 
tersects the plane of tbe ecliptic : 
one is termed the ascending, the 
other the descending, node, 221. 
Longitude of the node, the angular 
distance of the node from the first 
point of Aries. Line of, 221. 

Nubecula Major, 47. 

Nubecula Minor, 47. 

Nucleus (Lat. kernel), of a comet, 
300, 301 ; of sun-spots, 116. 

Nutation (nutatio, a nodding), an 
oscillatory movement of the Earth's 
axis, due to the Moon's attraction on 
the equatorial protuberance, 532. 

Object-glass of telescopes, 428; il- 
luminating power of, 429 ; accuracy 
required in constructing, 430 ; largest 
object-glass, 433. 

Obliquity of the ecliptic, variation 
of, 458. 

Occultation (oceultare, to hide), tbe 
eclipsing of a star or planet by the 
Moon or a planet. 

Opposition. A superior planet is in 
opposition wben the Sun, Earth, and 
the planet, are in the same straight 
line, with the Eartb in the middle, 
370. 

Optical double stars, 69. 

Orbit (orbis, a circle), the path of a 
planet or comet round the Sun, or of 
a satellite round a primary; of the 
planets, 174; of the Earth, 176; of 
the Moon, 222 ; of Mercury, 258 ; of 
comets, 298; of meteors, 312 ; incli- 
nations and nodes of the planetary 
orbits, 376. 

Parabola, a section of a cone parallel 
to one of its sides, 513. 

Parabolic orbits of comets, 298. 

Parallactic inequality, an irregulari- 
ty in the Moon's motion, arising 
from the difference of the Sun's at- 
traction at aphelion and perihelion. 

Parallax (nap6.\kai-i<;, alternation), 
455 ; correction for, 454, 455 ; equa- 
torial horizontal, 455 ; of the Moon, 
470 ; of Mars, 471 ; of the Sun, old 
and new value, 472 ; of the stars, 473, 
474. 

Parallels of latitude, 167 ; of declina- 
tion, 328. 



308 



ALPHABETICAL INDEX. 



Penumbra (pcene, almost, and um- 
bra, a shadow), the half-shadow which 
surrounds the deeper shadow in an 
eclipse, 240 ; of sun-spots, 116. 

Perigee (irepi, near, and yrj, the Earth). 
(1) The point in the Moon's orbit 
nearest the Earth, 218. (2) The posi- 
tion in which the Sun or other body 
is nearest the Earth. 

Perihelion (wept, near, and ijAios, the 
Sun), the point in an orbit nearest 
the Sun, 175; distance, the distance 
of a heavenly body from the Sun at 
its nearest approach; longitude of, 
one of the elements of an orbit, the 
angular distance of the perihelion 
point from the first point of Aries ; 
passage, the time at which a heaven- 
ly body makes its nearest approach 
to the Sun ; distance of comets, 298 ; 
of planets, see Appendix, Table n. ; 
change of, 407. 

Peri- Jove, Satumium, etc., the near- 
est approach of a satellite to Jupiter, 
Saturn, etc. 

Period (nepl, round, and 656s, a path), 
or periodic time, the time of a plan- 
et's, comet's, or satellite's revolu- 
tion; synodic, the time in which a 
planet returns to the same position 
with regard to the Sun and Earth, 
375. 

Perturbations {perturbare, to inter- 
fere with), the effects of the attrac- 
tions of* the planets and satellites 
upon each other, consisting of varia- 
tions in their motions and orbits, 
525, 526. 

Phases (#ao-c.s, an appearance), the 
various appearances presented by 
the illuminated portions of the Moon 
(235), and inferior planets (257, 262, 
369) in different parts of their orbits. 

Photography (<££?, light, and ypa.<i>rj, 
a painting), celestial, 495. 

Photosphere (<Ht6s, of light, and 
<r</>aipa, a sphere), of the stars, 82, 83 ; 
of the Sun, 124. 

Physical constitution, of the stars, 
82-84 ; of the Sun, 126. 

Plane, defined, 28 ; of the ecliptic, 111, 
145. 

Planet (nXavrJTris, a wanderer), a cool 
body revolving round a central in- 
candescent one. Planets change 
their positions with regard to the 



stars, 5 ; what they are, 11 ; names 
and symbols of, 138 ; explanation of 
symbols, 139; historical details, 140; 
a suspected planet, 143 ; travel round 
the Sun in elliptical orbits, 144 ; their 
orbits lie nearly in the plane of the 
ecliptic, 145 ; motions of, 147 ; dis- 
tances from the Sun, 148 ; periods of 
revolution, 148 ; diameters of, 150 ; 
comparative size of, 151 ; mass, vol- 
ume, and density, 154-157 ; minor, 
290-296 ; apparent movements of, 
364-374 ; varying distances from the 
Earth, 365-367 ; variations in size 
and brilliancy, 368 ; phases, 369 ; as- 
pects, 370 ; inferior and superior, 
256 ; conjunction and opposition, 370 ; 
elongations, 372 ; direct and retro- 
grade motion, and stationary points, 
373, 374; synodic periods, 375; in- 
clinations and nodes of orbits, 376 ; 
apparent paths among the stars, 379 ; 
elements of, see Appendix, Table II. 

Planetary nebulae, 97: 

Pleiades, a star-group, 86. 

Pointers, the, 342. 

Polar, diameter of the Earth, 162, 
171 ; circles, 170 ; compression of the 
Earth, explained, 202, 203 ; distance, 
329 ; axis of equatorial, 436. 

Polaris (Lat.), the pole-star, 341, 342 ; 
will not always mark the position of 
the north pole, 457. 

Poles (7ro\e'w, I turn), the extremities 
of the imaginary axis on which a 
celestial body rotates, 162 ; poles of 
the heavens, the extremities of the 
axis of the celestial sphere, 328 ; 
poles of the ecliptic, the extremities 
of the axis at right angles to the 
plane of the ecliptic, 360; of the 
Earth, 162; north celestial pole, 341 ; 
motion of, 457. 

Pores, on the solar surface, 122. 

Position -circle (of micrometers), 
439. 

Precession (prcecedere, to precede) 
of the equinoxes, a slow retrograde 
motion of the equinoctial points 
upon the ecliptic, 356, 457 ; cause of, 
explained, 528-530. 

Prime vertical, 333. 

Prisms (npl<rp.a), refract light, 413. 

Prominences, red, of the Sun, 123, 
251. 

Proper motion, see Motion. 



ALPHABETICAL INDEX. 



309 



Ptolemy, his theory of the solar sys- 
tem, 21 ; arranged the stars in 48 con- 
stellations, 54. 

Punctulations, on the solar surface, 
122. 

Pythagoras, taught that the planets 
revolve round the Sun, 21 ; divined 
the truth respecting the Milky Way, 
46. 

Quadrant {quadrant, a fourth part), 
the fourth part of the circumference 
of a circle, or 90° ; of altitude, a flex- 
ible strip of brass graduated into 90°, 
attached to the celestial globe, for 
determining celestial latitudes — dec- 
linations being determined by the 
brass meridian. 

Quadrature. Two heavenly bodies 
are said to be in quadrature when 
there is a difference of longitude of 
90° between them, 370. 

Quarters of the Moon, '235. 

Radiant-point of meteoric showers, 
311. 

Radiation, solar, 130. 

Radius (Lot. a spoke of a wheel) 
vector, an imaginary line joining 
the Sun and a planet or comet in any 
point of its orbit, 508. 

Rain, how caused, 211. 

Red-flames, and prominences, 123, 
251. 

Reflecting: telescope, 434 ; Earl of 
Rosse's, 435. 

Reflection, 411. 

Refracting: telescope, 428-433. 

Refraction {refrangere, to bend), at- 
mospheric, 411, 412; of light by 
prisms, 413 ; correction for, 451. 

Retrogradation, arc of, the arc ap- 
parently traversed by planets while 
their motion is retrograde, 373, 374. 

Retrograde motion, 373, 374. 

Revolution, the motion of one body 
round another ; time of, the period 
in which a heavenly body returns to 
the same point of its orbit. The 
revolution may either be anomalistic 
if measured from the aphelion or 
perihelion point, sidereal with refer- 
ence to a star, synodical with refer- 
ence to a node, or tropical with refer- 
ence to an equinox or tropic. 



Rig-lit Ascension, 228. 

Rilles, on the Moon, 231. 

Ring-s of Saturn, 282-287 ; why some- 
times invisible, 382. 

Rocks, stratified, 194 ; list of, 194 ; 
igneous, 197. 

Rotation, the motion of a body round 
a central axis : of Sun, 110 ; of Earth, 
177; of Moon, 234; of Earth, possibly 
slackening, 511. 

Rutherford, Mr., his lunar photo- 
graphs, 495. 



Saros, a term applied by the Chal- 
deans to the cycle of eclipses, 247. 

Satellites {satelles, a companion), the 
smaller bodies revolving round plan- 
ets and stars, 12, 146, 152 ; motions of, 
147 ; of Jupiter, 278 ; of Saturn, 281 ; 
of Uranus, 288 ; satellite of Neptune, 
289 ; elements of, see Appendix, Ta- 
ble m. 

Saturn, 280; distance from the Sun 
and period of revolution, 148 ; diame- 
ter, 150; volume, mass, and density, 
157 ; belts of, 281 ; satellites, 281 ; 
rings of, 282-287 ; dimensions of, 284; 
of what composed, 285 ; appearance 
of, from the body of the planet, 286 ; 
why sometimes invisible on the 
Earth, 382 ; atmosphere, 286 ; sea- 
sons, 286 ; solar eclipses due to the 
rings, 287 ; how presented to the 
Earth in different parts of its orbit, 
382. 

Schreiberzite, a mineral found in 
meteorites, 320. 

Scintillation {scintilla, a spark), the 
"twinkling" of the stars. 

Seasons of the Earth, 186-189 ; of 
Mercury, 259; of Venus, 264; of 
Mars, 272; of Jupiter, 274. 

Secular {seculum, an age), inequali- 
ties, .525; acceleration of the Moon's 
mean motion, 541. 

Selenography {o-e\rjvr), the Moon, 
and ypd<j)<o, I write), a description of 
the Moon. 

Semi-diurnal arc, see Arc. 

Sextant, an instrument consisting 
of the sixth part of a circle, finely 
graduated, with which the angular 
distances of celestial bodies are 
measured, 440. 

Shooting--stars, see Meteors. 



310 



ALPHABETICAL INDEX. 



Sidereal (sidus, a star), relating to 
the stars ; clock, 397 ; day, 396 ; time, 
397. 

Signs of the zodiac, 356. 

Sirius, its comparative brightness, 41. 

Snow on Mars, 271. 

Solar spectrum, 477. 

Solar system, 137-158. 

Solid, defined, 36. 

Solstices, or solstitial points (sol, the 
Sun, and stare, to stand still), the 
points in the Sun's path at which the 
extreme north and south declina- 
tions are reached, and at which the 
motion is apparently arrested before 
its direction is changed, 183 ; the 
Earth as seen from the Sun at the, 
188. 

Solstitial colure, see Colure. 

Spectroscope (spectrum, and o-Korrew, 
I see), 492 ; experiments with, 480 ; 
the Kew spectroscope, 493; direct 
vision, 494. 

Spectrum, 414 ; the solar, 477 ; grad- 
ual formation of, 478 ; dark lines and 
bright lines, 479, 480 ; spectrum anal- 
ysis, general laws of, 482 ; impor- 
tance of, 485 ; spectra of the stars, 
484 ; of nebulae, 486 ; of Moon and 
planets, 489 ; solar, stellar, and neb- 
ular spectra, illustrated, see Frontis- 
piece. 

Sphere (o-^aipa), defined, 37; celes- 
tial, the sphere of stars which ap- 
parently encloses the Earth, 1, 326 ; 
of observation, 330. 

Spheroid (a^alpa, a sphere, and etSo?, 
form), the solid formed by the rota- 
tion of an ellipse on one of its axes ; 
it is oblate if it rotates on the minor 
axis, and prolate if it rotates on the 
major axis, 39. 

Spring 1 tides, 535. 

Star-showers, see Meteors. 

Stars, why invisible in daytime, 3 ; 
why they appear at rest, 8 ; why they 
shine, 10 ; distance of nearest, 15 ; 
their distance generally, 43, 44; mag- 
nitudes of, 40, 41 ; telescopic, 40 ; com- 
parative brightness of, 41 ; distances 
of, 43, 44 ; diameters of, 45 ; distribu- 
tion of, 48 ; divided into constellations, 
53-57 ; names of, 58, 60 ; the twenty 
brightest, 62 ; proper motion of, 63; 
apparent motion of, 65, 334-337; 
doxible and multiple, 66-70 ; variable, 



71-77 ; new or temporary, 76 ; the 
Sun a variable star, 125 ; colored, 78- 
80 ; colored double, 79 ; physical con 
stitution of, 82-84; groups and clus- 
ters of, 86-88 ; nebulous, 98 ; appar- 
ent movements of, 334 ; apparent 
daily movements, 335-337; apparent 
yearly movements, 345, 346 ; visible 
in different latitudes, 338, 339 ; pole- 
• star, 341, 342; those seen at mid- 
night are opposite to the Sun, 346 ; 
how to identify the, 347; constella- 
tions visible throughout the year, 
349-351; circumpolar, 338; sidereal 
day, 353 ; catalogues, 449 ; spectra of, 
484 ; how the elements in the stars 
are determined, 485 ; parallax of the 
stars, 473, 474. 
Stationary - points, points in a 
planet's orbit at which it appears to 
have no motion among the stars, 373, 
374. 
Stones, meteoric, 317. 
Styles, old and new, 404; of sun-dials, 

387. 
Sun, is a star, 9; why it shines, 10; 
its relative brilliancy, 41, 106; ap- 
proaching the constellation Her- 
cules, 64; its disk, 105; distance, 
107 ; diameter, 108 ; volume and 
mass, 109 ; rotation, 110 ; inclination 
of axis, 112 ; sjin-spots, proper mo- 
tion of, 113 ; description of, 115-118 ; 
size of, 125 ; period of, 125 ; tele- 
scopic appearance of, 114-123 ; photo- 
sphere, 116, 124 ; atmosphere, 124, 126 ; 
faculae, 119; corrugations, or gran- 
ules. 120; willow-leaves, 121; pores, 
or punctulations, 122; red -flames, 
123, 251 ; the Sun, a variable star, 
125; elements in the photosphere, 
128 ; how determined by spectrum 
analysis, 483; benign influences of, 
127 ; light, 128 ; heat, 129 ; chemical 
force, 131; habitability, 134; future 
of, 135; eclipses of, 243-245; their 
phenomena, 248-252 ; solar heat, how 
accounted for by some, 313; apparent 
motions, 352 ; solar day, 353 * motion 
in the ecliptic, 358 ; rising, setting, 
and apparent daily path, 358 ; daily 
motion, explained with the celestial 
globe, 360; mean Sun, 390-393; irreg- 
ularities of the Sun's apparent daily 
motion, 391; distance, how deter- 
mined, 472 ; parallax of, old and new 



ALPHABETICAL INDEX. 



311 



value, 472; elements of, see Appen- 
dix, Table IV. 

Sun-dial, the, 385-387. 

Superior Conjunction, 370. 

Superior Planets, 256. 

Surface, defined, 28 plane, convex, 
and concave, defined, 28. 

Symbols {<rvy.&o\ov), signs used as 
abbreviations, see Appendix, Table I. 

Synodic period, 375. 

Syzygies (aw, with, and £vy6v, a 
yoke), the points in the Moon's orbit 
at which it is in a line with the Earth 
and Sun, or when it is in conjunction 
or opposition. 



Tails of comets, 300, 301. 

Tangent, defined, 81. 

Telescope (t^A.6, afar, and o-K<m£u>, I 
see), history, 427 ; construction, 428 ; 
illuminating or space - penetrating 
power, 99, 429 ; magnifying power, 
430 ; eye - pieces, 428, 431 ; object- 
glass, 428-430; tube, 432; powers of, 
433, 435 ; largest refractor, 433 ; re- 
flecting, 434; largest reflector, 435; 
various mountings, 436; equatorial, 
436 ; determination of positions with, 
448 ; altazimuth, 442 ; transit-circle, 
443-447; transit-instrument, 464. 

Temperature, of the Sun, 129 ; of the 
Earth's crust, 198, 199. 

Temporary Stars, 76. 

Terminator, 228. 

Thales, taught that the world was 
round, 21 ; predicted a memorable 
eclipse, 254. 

Theophrastus, his opinion respect- 
ing the Milky Way, 46. 

Tides (Saxon tidan. to happen), 534; 
spring and neap, 535; how produced, 
536-538 ; velocity and height of tidal 
wave, 540 ; effect of tidal action on 
Earth's rotation, 541. 

Time, as measured by the Sun, differs 
in different longitudes, 190; how to 
convert difference of time to differ- 
ence <5f longitude, 191 ; how meas- 
ured, 383 ; the mean Sun, motion of, 
393; equation of time, 394; sidereal, 
397; week, 398; month, 399; year, 
401 ; bissextile, 403 ; Julian and Gre- 
gorian calendar, 403, 404; how de- 
termined, 464; table of, see Appen- 
dix, Table VI. 



Trade-winds, 207-209. 

Transit (trans, across, and ire, to go), 
the passage (1) of a heavenly body 
across the meridian of a place (in the 
case of circumpolar stars there is an 
upper and a lower transit, the latter 
sometimes called the transit sub polo) ; 
(2) of one heavenly body across the 
disk of another, e. g., the transit of 
Venus across the Sun, 371; of a sat- 
ellite of Jupiter across the planet's 
disk, 278 ; methods of determining the 
time of, 447. 

Transit-circle, when used and gen- 
eral description of, 443 ; determina- 
tion of positions with, 444-446; de- 
termination of the time of transit, 
447. 

Transit-instrument, 464. 

Triangle, defined, 30. 

Tropics (TpeVw, I turn) of Cancer 
and Capricorn, the circles of decli- 
nation which mark the most north- 
erly and southerly points in the eclip- 
tic, in which the Sun occupies the 
signs named, 170. 

Tycno Brahe, added two constella- 
tions, 54 ; his clock, 389. 



Ultra - zodiacal planets, a name 
sometimes given to the minor plan- 
ets, because their orbits exceed the 
limits of the zodiac. ■ 

Umbra (Lat. a shadow), the darkest 
central portion of the shadow cast by 
a heavenly body, such as the Moon 
or Earth, 240 ; of sun-spots, 116. 

Universe, our, one of many, 8; shape 
of, 49-51. 

Uranus, 288 ; discovered by Sir Wm, 
Herscbel, 140 ; distance from the Sun 
and period of revolution, 148; diam- 
eter, 150 ; volume, mass, and density, 
157 ; satellites of, 288 ; specific grav- 
ity of, 288. 



Vapor, aqueous, 215. 

Variable, stars, 71-77 ; nebulae, 100. 

Variation of the Moon, one of the 
lunar inequalities. 

Ventis, 262; distance from the Sun 
and period of revolution, 148 ; diam- 
eter, 150; volume, mass, and den- 
sity, 157 ; polar compression, 264 ; its 



312 



ALPHABETICAL INDEX. 



phases, 262 ; seasons, 264 ; heat and 
light, 265; mountains, 265; a morn- 
ing and evening star by turns, 263 ; 
path of, among the stars, 378 ; tran- 
sits of, across the Sun's disk, 371, 
472. 

Verniers, 438. 

"Vertical (vertex^ the top). A vertical 
line (331) is a line perpendicular to 
the surface of the Eai-th at any place, 
and is directed therefore to the ze- 
nith ; a vertical circle is one that pass- 
es through the zenith and nadir of 
the celestial sphere ; the prime ver- 
tical (333) is the vertical circle pass- 
ing through the east and west points 
of the horizon. 

Via Lactea (Lat), see Milky Way. 

Volcanoes, of the Moon, 227. 

Volume (volumen, bulk), the cubical 
contents of a celestial body ; of the 
Sun, 109 ; of the planets, 157. 

Vulcan, a suspected planet, 143. 

Walled Plains, on the Moon. 231. 



Week, names of the days of, 398. 
Weight, what it is, 498. 
Willow-leaves in the penumbra of 

sun-spots, 121. 
Winds, 206-210. 



Year, the, 401 ; length of the sidereal 
and other years, 402 ; change in the 
length of solar, 406; length of the 
planets' years, see Appendix, Table 
II. 



Zenith, the point of the celestial 
sphere overhead, 327 ; distance, 332. 

Zodiac, the portion of the heavens 
extending 9° on either side of the 
ecliptic, in which the Sun and major 
planets appear to perform their an- 
nual revolutions, 356. 

Zodiacal, light, 138 ; its shape, 307 ; 
how explained, 307; constellations, 
55. 

Zones, torrid, frigid, and temperate, 
170. 



THE END. 




, CelesUctl Chart 

of ?Ju> Souf/iem Hejiz^phere 
Showing the position of 
t/ie Constellations and 'the 
nw&t important stars. 



Celestial CJiart 
fthe.Xorthem Hem 

~i*ff the position 
'■ Motion* and 
important stars 




Celestial Chart 
if t tie Southern Hem ■ (sphere- 

AVioiriiuj ttu position of 
Mi C'ansi-ellatifiiiA and l/u 

UtOH'i I mpOI'ti i ,1 / .;;', //:;: 



T5 



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